[PATCH] msvcrt: Import memmove from musl

[Gabriel Ivăncescu]

FWIW "rep movsb" is supposedly the fastest when transferring larger 
blocks (I think more than 128 bytes?) on recent CPUs. The cool thing is 
that the CPU handles everything, no matter the alignment or "memcpy vs 
memmove", so it's by far the simplest, and since it knows about the 
alignment requirements of that particular CPU it can optimize it 
internally itself.

Same story with "rep stosb" for memset. Unfortunately these are very 
slow on older CPUs. I think there's a CPUID flag that says whether they 
are fast, we could use that.