illumos should put libc last in library search order

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -340,6 +340,14 @@ impl<'a> Linker for GccLinker<'a> {
     }
 
     fn link_dylib(&mut self, lib: Symbol, verbatim: bool, as_needed: bool) {
+        if self.sess.target.os == "illumos" && lib.as_str() == "c" {
+            // libc will be added via late_link_args on illumos so that it will
+            // appear last in the library search order.
+            // FIXME: This should be replaced by a more complete and generic
+            // mechanism for controlling the order of library arguments passed
+            // to the linker.
+            return;
+        }
         if !as_needed {
             if self.sess.target.is_like_osx {
                 // FIXME(81490): ld64 doesn't support these flags but macOS 11

compiler/rustc_target/src/spec/illumos_base.rs

@@ -6,6 +6,17 @@ pub fn opts() -> TargetOptions {
     late_link_args.insert(
         LinkerFlavor::Gcc,
         vec![
+            // The illumos libc contains a stack unwinding implementation, as
+            // does libgcc_s.  The latter implementation includes several
+            // additional symbols that are not always in base libc.  To force
+            // the consistent use of just one unwinder, we ensure libc appears
+            // after libgcc_s in the NEEDED list for the resultant binary by
+            // ignoring any attempts to add it as a dynamic dependency until the
+            // very end.
+            // FIXME: This should be replaced by a more complete and generic
+            // mechanism for controlling the order of library arguments passed
+            // to the linker.
+            "-lc".to_string(),
             // LLVM will insert calls to the stack protector functions
             // "__stack_chk_fail" and "__stack_chk_guard" into code in native
             // object files.  Some platforms include these symbols directly in