treewide: fixes to allow x64 darwin to default to sdk 11

update code to not assume that x64 darwin must use sdk 10.12. After this
change it's possible to build a sdk 11 stdenv on darwin x64

pkgs/os-specific/darwin/apple-sdk-11.0/default.nix

@@ -51,15 +51,15 @@ let
   };
 
   mkCc = cc:
-    if stdenv.isAarch64 then cc
+    if lib.versionAtLeast stdenv.hostPlatform.darwinSdkVersion "11" then cc
     else
       cc.override {
         bintools = stdenv.cc.bintools.override { libc = packages.Libsystem; };
         libc = packages.Libsystem;
       };
 
   mkStdenv = stdenv:
-    if stdenv.isAarch64 then stdenv
+    if lib.versionAtLeast stdenv.hostPlatform.darwinSdkVersion "11" then stdenv
     else
       let
         darwinMinVersion = "10.12";

pkgs/os-specific/darwin/apple-source-releases/default.nix

@@ -1,6 +1,10 @@
 { lib, stdenv, fetchurl, fetchFromGitHub, pkgs }:
 
 let
+
+  isSdk10_12 = stdenv.hostPlatform.darwinSdkVersion == "10.12";
+
+
   # This attrset can in theory be computed automatically, but for that to work nicely we need
   # import-from-derivation to work properly. Currently it's rather ugly when we try to bootstrap
   # a stdenv out of something like this. With some care we can probably get rid of this, but for
@@ -271,10 +275,10 @@ developerToolsPackages_11_3_1 // macosPackages_11_0_1 // {
     Libinfo         = applePackage "Libinfo"           "osx-10.11.6"     "sha256-6F7wiwerv4nz/xXHtp1qCHSaFzZgzcRN+jbmXA5oWOQ=" {};
     Libm            = applePackage "Libm"              "osx-10.7.4"      "sha256-KjMETfT4qJm0m0Ux/F6Rq8bI4Q4UVnFx6IKbKxXd+Es=" {};
     Libnotify       = applePackage "Libnotify"         "osx-10.12.6"     "sha256-6wvMBxAUfiYcQtmlfYCj1d3kFmFM/jdboTd7hRvi3e4=" {};
-    libmalloc       = if stdenv.isx86_64 then
+    libmalloc       = if isSdk10_12 then
       applePackage "libmalloc" "osx-10.12.6" "sha256-brfG4GEF2yZipKdhlPq6DhT2z5hKYSb2MAmffaikdO4=" {}
     else macosPackages_11_0_1.libmalloc;
-    libplatform     = if stdenv.isx86_64 then
+    libplatform     = if isSdk10_12 then
       applePackage "libplatform"       "osx-10.12.6"     "sha256-6McMTjw55xtnCsFI3AB1osRagnuB5pSTqeMKD3gpGtM=" {}
     else macosPackages_11_0_1.libplatform;
     libpthread      = applePackage "libpthread"        "osx-10.12.6"     "sha256-QvJ9PERmrCWBiDmOWrLvQUKZ4JxHuh8gS5nlZKDLqE8=" {};
@@ -286,7 +290,7 @@ developerToolsPackages_11_3_1 // macosPackages_11_0_1 // {
     objc4           = applePackage "objc4"             "osx-10.12.6"     "sha256-ZsxRpdsfv3Dxs7yBBCkjbKXKR6aXwkEpxc1XYXz7ueM=" {};
     ppp             = applePackage "ppp"               "osx-10.12.6"     "sha256-M1zoEjjeKIDUEP6ACbpUJk3OXjobw4g/qzUmxGdX1J0=" {};
     removefile      = applePackage "removefile"        "osx-10.12.6"     "sha256-UpNk27kGXnZss1ZXWVJU9jLz/NW63ZAZEDLhyCYoi9M=" {};
-    xnu             = if stdenv.isx86_64 then
+    xnu             = if isSdk10_12 then
       applePackage "xnu" "osx-10.12.6" "sha256-C8TPQlUT3RbzAy8YnZPNtr70hpaVG9Llv0h42s3NENI=" {}
     else macosPackages_11_0_1.xnu;
     hfs             = applePackage "hfs"               "osx-10.12.6"     "sha256-eGi18HQFJrU5UHoBOE0LqO5gQ0xOf8+OJuAWQljfKE4=" {};
@@ -297,7 +301,7 @@ developerToolsPackages_11_3_1 // macosPackages_11_0_1 // {
     diskdev_cmds    = applePackage "diskdev_cmds"      "osx-10.11.6"     "sha256-VX+hcZ7JhOA8EhwLloPlM3Yx79RXp9OYHV9Mi10uw3Q=" {
       macosPackages_11_0_1 = macosPackages_11_0_1;
     };
-    network_cmds    = if stdenv.isx86_64 then
+    network_cmds    = if isSdk10_12 then
       applePackage "network_cmds" "osx-10.11.6" "sha256-I89CLIswGheewOjiNZwQTgWvWbhm0qtB5+KUqzxnQ5M=" {}
     else macosPackages_11_0_1.network_cmds;
     file_cmds       = applePackage "file_cmds"         "osx-10.11.6"     "sha256-JYy6HwmultKeZtLfaysbsyLoWg+OaTh7eJu54JkJC0Q=" {};

pkgs/os-specific/darwin/apple-source-releases/xnu/default.nix

@@ -12,7 +12,7 @@ appleDerivation' (if headersOnly then stdenvNoCC else stdenv) (
 
   nativeBuildInputs = [ bootstrap_cmds bison flex gnum4 unifdef perl python3 ];
 
-  patches = lib.optionals stdenv.isx86_64 [ ./python3.patch ];
+  patches = lib.optionals (lib.versionOlder stdenv.hostPlatform.darwinSdkVersion "11") [ ./python3.patch ];
 
   postPatch = ''
     substituteInPlace Makefile \
@@ -48,7 +48,7 @@ appleDerivation' (if headersOnly then stdenvNoCC else stdenv) (
       --replace 'MACHINE_ARCH=armv7' 'MACHINE_ARCH=arm64' # this might break the comments saying 32-bit is required
 
     patchShebangs .
-  '' + lib.optionalString stdenv.isAarch64 ''
+  '' + lib.optionalString (lib.versionAtLeast stdenv.hostPlatform.darwinSdkVersion "11") ''
     # iig is closed-sourced, we don't have it
     # create an empty file to the header instead
     # this line becomes: echo "" > $@; echo --header ...
@@ -72,15 +72,19 @@ appleDerivation' (if headersOnly then stdenvNoCC else stdenv) (
   HOST_FLEX = "flex";
   HOST_BISON = "bison";
   HOST_GM4 = "m4";
-  MIGCC = "cc";
+  # use unwrapped clang to generate headers because wrapper is not compatible with a 32 bit -arch.
+  # aarch64 should likely do this as well and remove the --replace MACHINE_ARCH above
+  MIGCC = if stdenv.isx86_64 && lib.versionAtLeast stdenv.hostPlatform.darwinSdkVersion "11"
+    then "${lib.getBin buildPackages.stdenv.cc.cc}/bin/clang"
+    else "cc";
   ARCHS = arch;
   ARCH_CONFIGS = arch;
 
   env.NIX_CFLAGS_COMPILE = "-Wno-error";
 
   preBuild = let macosVersion =
     "10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11" +
-    lib.optionalString stdenv.isAarch64 " 10.12 10.13 10.14 10.15 11.0";
+    lib.optionalString (lib.versionAtLeast stdenv.hostPlatform.darwinSdkVersion "11") " 10.12 10.13 10.14 10.15 11.0";
    in ''
     # This is a bit of a hack...
     mkdir -p sdk/usr/local/libexec
@@ -150,7 +154,9 @@ appleDerivation' (if headersOnly then stdenvNoCC else stdenv) (
     mv $out/Library/Frameworks/IOKit.framework $out/Library/PrivateFrameworks
   '';
 
-  appleHeaders = builtins.readFile (./. + "/headers-${arch}.txt");
+  appleHeaders = let
+    name = if stdenv.hostPlatform.darwinSdkVersion == "10.12" then "headers-10.12-${arch}.txt" else "headers-${arch}.txt";
+  in builtins.readFile (./. + "/${name}");
 } // lib.optionalAttrs headersOnly {
   HOST_CODESIGN = "echo";
   HOST_CODESIGN_ALLOCATE = "echo";