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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

6807164500e9920638e2ab0cdb4bf8321d24f8eb is a good reference example on what adding a new header entails.

 The headers.txt files are what roots the generation of the headers into the build system.

You can for now skip additions to libc/docs/, since I don't think I properly handle in docgen what to do for BSD extensions (docgen will error IIRC if you add things that aren't std C or POSIX).

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