[llvm] [Support/BLAKE3] Make g_cpu_features thread safe (PR #147948)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur approved this pull request.

LGTM

@akyrtzi The LLVM repository only allows squashed merges. It even is encouraged to never force-push to a PR branch since it may cause GitHub to lose track of comments on source lines.

https://github.com/llvm/llvm-project/pull/147948