[OpenMP][DeviceRTL] Implement libc function `memcmp`

We will add some simple implementation of libc functions starting from
this patch, and the first one is `memcmp`, which is reported in #56929. Note that
`malloc` and `free` are not included in this patch because of the use of
`declare variant`. In the near future we will implement the two functions w/o
using any vendor provided function.

This fixes #56929.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D131182