[llvm] [SystemZ][z/OS] Add guard for SI_USER (PR #213101)

[Abhina Sree via llvm-commits]

abhina-sree wrote:

> If the signal came from another process -- i.e. `SI_ASYNCIO` or `si_code <= 0` -- we definitely want to reraise it. Unfortunately that documentation doesn't really clarify what happens in the case of `kill(getpid(), sig)`...

I agree, I'm still working on investigating what the proper fix should be. In the interim, is this patch ok to unblock our build? https://lab.llvm.org/staging/#/builders/237/builds/26

https://github.com/llvm/llvm-project/pull/213101