[libc] Adding a version of memcpy w/ software prefetching

For machines with a lot of cores, hardware prefetchers can saturate the memory bus when utilization is high.
In this case it is desirable to turn off the hardware prefetcher completely.
This has a big impact on the performance of memory functions such as `memcpy` that rely on the fact that the next cache line will be readily available.

This patch adds the 'LIBC_COPT_MEMCPY_X86_USE_SOFTWARE_PREFETCHING' compile time option that generates a version of memcpy with software prefetching. While not fully restoring the original performances it mitigates the impact to an acceptable level.

Reviewed By: rtenneti

Differential Revision: https://reviews.llvm.org/D154494