[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: https://github.com/google/jax/issues/5501

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