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

[Abhina Sree via llvm-commits]

abhina-sree wrote:

> Fairly unusual for `SI_USER` to not be defined. In fact, probably not POSIX-compliant. What does z/OS use when you use `kill(2)` to send a signal?

On z/OS, based on the documentation [here](https://www.ibm.com/docs/en/zos/3.1.0?topic=functions-sigaction-examine-change-signal-action) I think si_code will be less than or equal to zero if the signal was generated by another process. Then si_pid and si_uid will have the process id and user id respectively

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