[llvm-branch-commits] [compiler-rt] [msan] Use `pthread_atfork` instead of interceptor (PR #75398)

[Vitaly Buka via llvm-branch-commits]

https://github.com/vitalybuka created https://github.com/llvm/llvm-project/pull/75398

This is done for consistency with other sanitizers.
Also lock the allocator.