[kernel][libc] Select x86 memcpy/memset implementation at boot

This allows us to make use of ERMS on devices that support it, and
fallback to something more performant otherwise.

ZX-1395 #comment Done for kernel-mode
ZX-1346 #comment This provides runtime selection for memcpy/memset

Change-Id: I96e8ca96e17a677ffa95c59cd36682cb16f2904b