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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Why not update the entire BLAKE3? Wie generally do not want to fork third-party software, but update it [as the need arises](https://github.com/llvm/llvm-project/commit/54c1a9b20d89e85cd60d002c77b34c00f36520f4). Every LLVM-private change will make eventually updating it more difficult. This incliudes cherry-picked patches from the upstream repository. See also https://reviews.llvm.org/D121510#3383116



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