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

[Zibi Sarbinowski via llvm-commits]

zibi2 wrote:

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

However, this is internal change which eventually will go away.


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