[llvm] [Support] Don't re-raise signals sent from kernel (PR #145759)

[Alex Langford via llvm-commits]

bulbazord wrote:

> I assume that si_pid==0 is macOS kernel's behavior. Can you add some description whether it is also Linux's?

AFAICT this is the behavior for both macOS and Linux, not sure about other unix and unix-like OSes. Would you like me to add a comment here?

For completeness, I also considered looking at the `si_code` field because on Linux it has a `SI_KERNEL` value if it's sent from the kernel, but macOS doesn't seem to have an equivalent.

https://github.com/llvm/llvm-project/pull/145759