[llvm] Add sysexits.h header with BSD exit codes (total-18) (PR #126112)

[Nick Desaulniers via llvm-commits]

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

Ah, sorry, the intent of #124640 was to provide sysexits.h to llvm-libc, (lib/), not llvm itself.

Specifically, that build failure isn't llvm's fault, it's only when building llvm on top of the nascent llvm-libc is the deficiency in llvm-libc exposed.

https://github.com/llvm/llvm-project/pull/126112