[llvm] [SystemZ][z/OS] z/OS does not support nanosleep, use usleep instead (PR #109823)

[Zibi Sarbinowski via llvm-commits]

https://github.com/zibi2 requested changes to this pull request.

We do have nanosleep() under `std::__ibm`.

Note that this change is temporary only until we move up the LE level having this function.

https://github.com/llvm/llvm-project/pull/109823