Use native CC toolchains on Windows and macOS; restrict LLVM exec to Linux

The hermetic LLVM exec toolchains for Windows and macOS cause build failures:

- Windows: the minimal LLVM package ships only a GNU-mode clang++ that
  rejects MSVC-style linker flags (/NOLOGO, backslash paths, .lib files)
  generated by rustc for x86_64-pc-windows-msvc targets. Removing the
  Windows LLVM exec toolchain registrations and keeping the existing
  BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=0 override allows Bazel to detect
  Visual Studio's link.exe on Windows runners.

- macOS: the hermetic LLVM sysroot lacks compiler-rt builtins at the path
  aws-lc-sys's build script expects (configuration hash mismatch between the
  build-script exec config and the main build config). It also lacks system
  frameworks like CoreServices required by aws-lc-sys. Removing the macOS
  LLVM exec toolchain registration and re-enabling Xcode CC detection
  (BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=0, BAZEL_NO_APPLE_CPP_TOOLCHAIN=0)
  lets macOS runners use Xcode clang which has full system access.

LLVM exec toolchains are kept only for Linux where they work correctly and
are required for the musl cross-compilation targets.

Author: AutomatedTester

.bazelrc

@@ -16,11 +16,15 @@ common --lockfile_mode=off
 # resolution on developer hosts or CI runners.
 common --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
 common --repo_env=BAZEL_NO_APPLE_CPP_TOOLCHAIN=1
-# On Windows re-enable MSVC CC detection so the Visual Studio toolchain
-# (link.exe) is available for Rust MSVC targets. The hermetic LLVM package
-# for Windows only provides a GNU-mode clang++ which cannot handle the
-# MSVC-style linker flags that rustc emits for x86_64-pc-windows-msvc.
+# Re-enable native CC toolchain detection on Windows and macOS. The hermetic
+# LLVM exec toolchain is only registered for Linux (see MODULE.bazel).
+#   Windows: needs MSVC link.exe; the LLVM minimal package ships only a
+#     GNU-mode clang++ that rejects /NOLOGO and backslash paths.
+#   macOS: needs Xcode clang for compiler-rt builtins and system frameworks
+#     (e.g. CoreServices) required by aws-lc-sys.
 common:windows --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=0
+common:macos --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=0
+common:macos --repo_env=BAZEL_NO_APPLE_CPP_TOOLCHAIN=0
 
 # Bazel 8 compatibility flags. We should find ways to avoid needing these
 common --legacy_external_runfiles

MODULE.bazel

@@ -423,18 +423,14 @@ use_repo(rust_toolchains, "default_rust_toolchains")
 register_toolchains("@default_rust_toolchains//:all")
 
 llvm_toolchains = use_extension("@llvm//extensions:toolchain.bzl", "toolchain")
-llvm_toolchains.exec(
-    arch = "x86_64",
-    os = "windows",
-)
-llvm_toolchains.exec(
-    arch = "aarch64",
-    os = "windows",
-)
-llvm_toolchains.exec(
-    arch = "aarch64",
-    os = "macos",
-)
+# Windows and macOS use their native toolchains (MSVC / Xcode) so that:
+#   - Windows: link.exe handles the MSVC-ABI linker flags rustc generates for
+#     x86_64-pc-windows-msvc (the hermetic LLVM minimal package only ships a
+#     GNU-mode clang++ that rejects /NOLOGO and backslash paths).
+#   - macOS: Xcode clang has access to compiler-rt builtins and system
+#     frameworks (CoreServices etc.) that aws-lc-sys needs; the hermetic LLVM
+#     sysroot cannot satisfy those at compile time.
+# LLVM exec toolchains are registered only for Linux where they work correctly.
 llvm_toolchains.exec(
     arch = "x86_64",
     os = "linux",