[Bazel] Add support for targeting macOS arm64

In attempting to build JAX on Apple Silicon, we discovered an issue with
the bazel configuration in llvm-project-overlay. This patch fixes the
logic, at least when building JAX. More context is included on the
following GitHub issue: jax-ml/jax#5501

Differential Revision: https://reviews.llvm.org/D109839

Author: dfm

utils/bazel/llvm-project-overlay/llvm/config.bzl

@@ -76,7 +76,8 @@ os_defines = select({
 # TODO: We should split out host vs. target here.
 llvm_config_defines = os_defines + select({
     "@bazel_tools//src/conditions:windows": native_arch_defines("X86", "x86_64-pc-win32"),
-    "@bazel_tools//src/conditions:darwin": native_arch_defines("X86", "x86_64-unknown-darwin"),
+    "@bazel_tools//src/conditions:darwin_arm64": native_arch_defines("AArch64", "arm64-apple-darwin"),
+    "@bazel_tools//src/conditions:darwin_x86_64": native_arch_defines("X86", "x86_64-unknown-darwin"),
     "@bazel_tools//src/conditions:linux_aarch64": native_arch_defines("AArch64", "aarch64-unknown-linux-gnu"),
     "//conditions:default": native_arch_defines("X86", "x86_64-unknown-linux-gnu"),
 }) + [