Simplify MACHINE_ARCH to be a single string.

Big endian and armv4 mean that we are now down to only two supported
variants. A future change will use MACHINE_ARCH in assembly which
does not support C-style string concatentation and thus needs
MACHINE_ARCH defined as a single string.

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25211