[llvm] Enable using threads on z/OS (PR #171847)

[Petr Penzin via llvm-commits]

ppenzin wrote:

I've tried to switch from thread::id to plain integers and got redefinition errors for affected functions instead. I am also not sure that my attempts to fix this won't break z/OSS builds instead. Also, completely agree with @jrtc27 that would be great to remove this set up, but in a meantime if we want to revert this patch I have opened #177464

https://github.com/llvm/llvm-project/pull/171847