Remove the old ARMv4 memcpy

This was only used when building for ARMv4 or some ARMv5 or when
_STANDALONE is defined. As ARMv4 and ARMv5 support has been removed,
and we only define _STANDALONE in the bootloader where we don't use
this version of memcpy we can remove it.

Differential Revision: https://reviews.freebsd.org/D28313