Stop emitting the dso_local LLVM attribute for external symbols under the static relocation model on macOS.

This matches Clang's behavior:

973cb2c326/clang/lib/CodeGen/CodeGenModule.cpp (L1038-L1040)

Even if `dso_local` were properly supported in this way on macOS, it seems
incorrect to add this annotation as liberally as we did. The `dso_local`
annotation is for symbols that ultimately end up in the same linkage unit, but
we were adding this annotation even for `static` values inside `extern` blocks
marked with `#[link(type="framework")]`, which should be considered dynamically
linked.  Note that Clang likewise avoids emitting `dso_local` for `dllimport`
symbols:

973cb2c326/clang/lib/CodeGen/CodeGenModule.cpp (L1005-L1007)

This issue caused breakage in the `ring` crate, which links to a symbol defined
in `Security.framework` that ultimately resolves to address `0x0`:

b94d61e044/src/rand.rs (L390)

For this symbol, the use of `dso_local` causes LLVM to emit a relocation of
type `X86_64_RELOC_SIGNED`, which is a 32-bit signed PC-relative offset. If the
binary is large enough, `0x0` might be out of range, and the link will fail.
Avoiding `dso_local` causes LLVM to use the GOT instead, emitting a relocation
of type `X86_64_RELOC_GOT_LOAD`, which will properly handle the large offset
and cause the link to succeed.

As a side note, the static relocation model is effectively deprecated for
security reasons on macOS, as it prohibits PIE. It's also completely
unsupported on Apple Silicon, so I don't think it's worth going to the effort
of properly supporting this model on that platform.

compiler/rustc_codegen_llvm/src/mono_item.rs

@@ -135,6 +135,11 @@ impl CodegenCx<'ll, 'tcx> {
             return false;
         }
 
+        // Match clang by only supporting COFF and ELF for now.
+        if self.tcx.sess.target.is_like_osx {
+            return false;
+        }
+
         // Static relocation model should force copy relocations everywhere.
         if self.tcx.sess.relocation_model() == RelocModel::Static {
             return true;