Create one list of replacements for MACHINE_CPUARCH as MACHINE_CPUARCH_SUB.

This also adds missing s/aarch64/arm64 to the sys.mk version and also
adds back armv6hf for universe since it was added to the sys.mk version
in r300438.

MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D7159