[AArch64] Optimized memcpy.

[AArch64] Optimized memcpy.

This is an optimized memcpy for AArch64.  Copies are split into 3 main
cases: small copies of up to 16 bytes, medium copies of 17..96 bytes
which are fully unrolled.  Large copies of more than 96 bytes align
the destination and use an unrolled loop processing 64 bytes per
iteration.  In order to share code with memmove, small and medium
copies read all data before writing, allowing any kind of overlap.  On
a random copy test memcpy is 40.8% faster on A57 and 28.4% on A53.