[libc] add strncmp to strings

Add strncmp as a function to strings.h. Also adds unit tests, and adds
strncmp as an entrypoint for all current platforms.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D106901

GitOrigin-RevId: c6d03b583b48c00171e79d3bcf61168c97f874b9
8 files changed