Consolidate handling of libdl, and handle crt-static

Move the link line for `libdl` up to `src/unix/mod.rs`, making it easier
to see all the libraries `libc` links to.

This also makes `libdl` respect `target-feature=+crt-static`.

src/unix/linux_like/linux/gnu/mod.rs

@@ -1435,7 +1435,6 @@ extern "C" {
     ) -> ::c_int;
 }
 
-#[link(name = "dl")]
 extern "C" {
     pub fn dlmopen(
         lmid: Lmid_t,

src/unix/mod.rs

@@ -303,6 +303,8 @@ cfg_if! {
                         feature = "rustc-dep-of-std"))] {
         #[link(name = "c", kind = "static-nobundle",
             cfg(target_feature = "crt-static"))]
+        #[link(name = "dl", kind = "static-nobundle",
+            cfg(target_feature = "crt-static"))]
         #[link(name = "m", kind = "static-nobundle",
             cfg(target_feature = "crt-static"))]
         #[link(name = "pthread", kind = "static-nobundle",
@@ -312,6 +314,7 @@ cfg_if! {
         #[link(name = "util", kind = "static-nobundle",
             cfg(target_feature = "crt-static"))]
         #[link(name = "c", cfg(not(target_feature = "crt-static")))]
+        #[link(name = "dl", cfg(not(target_feature = "crt-static")))]
         #[link(name = "m", cfg(not(target_feature = "crt-static")))]
         #[link(name = "pthread", cfg(not(target_feature = "crt-static")))]
         #[link(name = "rt", cfg(not(target_feature = "crt-static")))]