Support for bazel builds on macos_arm64

[simonmaurer]

with respect to this issue this PR allows bazel builds for the recent macos_arm64 platform.

verified using the following bazel versions:

• bazel-3.7.2-darwin-x86_64
• bazel-4.0.0-darwin-x86_64
• bazel-4.0.0-homebrew
• bazel-3.7.1-darwin-arm64

bazel -c opt --config=macos_arm64 :end2end_bench

specifying --config=macos_arm64 is mandatory to allow a succesful build of the XNNPACKlibrary
if we wanna build the ènd2end_bench another PR has to be made to track the latest commit of pthreadpool(once this PR is accepted)

as TensorFlow depends on XNNPACK, once this PR is accepted we can issue a PR on the official TensorFlow repo. this will allow bazel builds of the TensorFlow Lite library and the benchmark_model for platform macos_arm64

[simonmaurer]

updated pthreadpool library, using latest commit which pulls in PR #14

end2end_bench can now be built using:

bazel -c opt --config=macos_arm64 :end2end_bench

[simonmaurer]

@Maratyszcza thank you for reviewing!