wip macos

motiejus@mtwork:/code/bazel-zig-cc$ bazel build  --spawn_strategy=local  --platforms @zig_sdk//libc_aware/platform:macos_aarch64_sdk.13.1 test/c:which_libc
    INFO: Analyzed target //test/c:which_libc (0 packages loaded, 0 targets configured).
    INFO: Found 1 target...
    Target //test/c:which_libc up-to-date:
      bazel-bin/test/c/which_libc
    INFO: Elapsed time: 0.071s, Critical Path: 0.00s
    INFO: 1 process: 1 internal.
    INFO: Build completed successfully, 1 total action
    motiejus@mtwork:/code/bazel-zig-cc$ file bazel-bin/test/c/which_libc
    bazel-bin/test/c/which_libc: Mach-O 64-bit arm64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|PIE>
    motiejus@mtwork:/code/bazel-zig-cc$

toolchain/platform/defs.bzl

@@ -14,7 +14,7 @@ def declare_platforms():
             for os in oss:
                 declare_platform(gocpu, zigcpu, bzlos, os)
 
-def declare_libc_aware_platforms():
+def declare_libc_aware_platforms(macos_sdk_versions):
     # create @zig_sdk//{os}_{arch}_platform entries with zig and go conventions
     # with libc specified
     for zigcpu, gocpu in _CPUS:
@@ -27,6 +27,15 @@ def declare_libc_aware_platforms():
                 suffix = "_{}".format(libc),
                 extra_constraints = ["@zig_sdk//libc:{}".format(libc)],
             )
+        for macos_sdk_version in macos_sdk_versions:
+            declare_platform(
+                gocpu,
+                zigcpu,
+                "macos",
+                "macos",
+                suffix = "_sdk.{}".format(macos_sdk_version),
+                extra_constraints = ["@zig_sdk//libc:macos.{}".format(macos_sdk_version)],
+            )
 
 def declare_platform(gocpu, zigcpu, bzlos, os, suffix = "", extra_constraints = []):
     constraint_values = [