Add an elf note on ARM to store the MACHINE_ARCH an executable was built

Add an elf note on ARM to store the MACHINE_ARCH an executable was built
for. This is useful for software needing to know which architecture a
binary is built for as arm and armv6 have slight differences meaning only
some binaries build for one will work as expected on the other. It is
expected pkgng will be able to make use of this to simplify the logic to
determine which package ABI to use.

Approved by:	re (kib)