arm64: speed up simple memcpy/memset alternatives

We need those simple alternatives to be used during early boot when the
MMU is not yet enabled. However they don't have to be the slowest they
can be. Those functions are mainly used to clear .bss sections and copy
.data to final destination when doing XIP, etc. Therefore it is very
likely for provided pointers to be 64-bit aligned. Let's optimize for
that case.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>