bsd.compat.mk: Set MACHINE before including bsd.opts.mk

In bsd.opts.mk we check MACHINE_ARCH and may want to check MACHINE to
decide which options to enable. Unfortunately this is included too
early via bsd.compiler.mk.

Move including bsd.compiler.mk until after we can set MACHINE and
MACHINE_ARCH.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42448