Add a USE_GCC_TOOLCHAINS knob to make universe.

This uses GCC toolchains instead of LLVM on architectures supported by
GCC. Currently this uses GCC 6 on aarch64, amd64, i386, and mips.

Although this does also try to use GCC 6 on powerpc, it is always
skipped for now since a powerpc-gcc6 package is not available and the
structure of make universe makes it hard to skip a subset of arches
for a target. This should be short-lived as freebsd-gcc9 does include
a powerpc-gcc9 package so powerpc should work once this switches to
GCC 9.

Discussed with: emaste
Differential Revision: https://reviews.freebsd.org/D25732