[llvm] [bazel] Port #145358: LIBC_THREAD_MODE (PR #151539)

[William Huynh via llvm-commits]

https://github.com/saturn691 approved this pull request.

Hmm, it should default to `LIBC_THREAD_MODE_PLATFORM`...

https://github.com/llvm/llvm-project/blob/7abb519e9ecf9368dbc4f346830d786f4638edae/libc/config/config.json#L8-L13

https://github.com/llvm/llvm-project/pull/151539