[libc][NFC] Split definitions of memcpy and memmove

[gfunni234_gmail.com]

Summary

Memmove is a valid way to implement memcpy. However, memcpy isn’t a valid way to implement memmove.

For this reason, I split the definitions into two. No ABI breakage should occur

[jrtc27]

Your description is not what this does for userspace. It is what it does for the kernel but that is bad, now we have two copies of basically the same thing in one file.