[libcxx] [libcxxabi] [llvm] [libc++] Implement library support for MCF thread model (PR #116550)

[A. Jiang via llvm-commits]

frederick-vs-ja wrote:

We can rebase the patch now as the test is (temporarily?) disabled for MSan by #116933.

https://github.com/llvm/llvm-project/pull/116550