[libomptarget][OpenMP] Initial implementation of omp_target_memset() and...

[libomptarget][OpenMP] Initial implementation of omp_target_memset() and omp_target_memset_async() (#68706)

Implement a slow-path version of omp_target_memset*() 

There is a TODO to implement a fast path that uses an on-device
kernel instead of the host-based memory fill operation.  This may
require some additional plumbing to have kernels in libomptarget.so