compat: Allow explicit overriding of COMPAT_ARCH and COMPAT_CPUTYPE

Summary:
Allow src.conf to override the inferred COMPAT_ARCH and COMPAT_CPUTYPE
variables, such that a different CPU target can be specified explicitly
for the general target vs the compat target.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D23992