Import the arm64 Cortex Strings memset into the kernel

Bring in the last version of the optimized memset from the Cortex
Strings library that didn't use the VFP registers. While here clean up
to use the ENTRY/EDIT macros.

This is imported from:
https://git.linaro.org/toolchain/cortex-strings.git/tree/src/aarch64/memset.S?id=bc654e7a594385071b42db3c9c94bd0fd5fe0da5

Sponsored by: The FreeBSD Foundation