Disable rarely used architecture variants in make universe by default.

If EXTRA_TARGETS is defined, build all supported architecture
variants. By default, build architecture variants needed to provide
code coverage or that are commonly used.

Use this to disable building of all the hard-float and little-endian
MIPS architecture variants along with n32 by default.

Reviewed by: rpokala
Discussed with: imp, emaste
Differential Revision: https://reviews.freebsd.org/D24178