[kernel][arm64] Optimized arm64 memcpy, memset

These are taken verbatim from Linaro's cortex-strings. memset is
from an older version of cortex-strings that does not use NEON. The
current cortex-strings memcpy (also now used in libc) never used NEON.

MG-526 #done

Change-Id: Ic01878a1c702c3677103e5db0c67ce114e201183