Don't compile old gcc 4.2.1 archs by default in universe/tinderbox.

Only compile clang supporting architectures of amd64, arm, arm64,
i386, and riscv as part of universe. Compile the other architectures
if MAKE_OBSOLETE_GCC is defined. In all cases, explicit lists of
architectures in TARGETS= on the command line override.

For mips, powerpc and sparc64, do the same thing we do for risvc when
MAKE_OBSOLETE_GCC isn't defined and short-circuit their universe build
with an echo saying to install the xtoolchain port or pkg.

PR: 241134
Discussed on: arch@ (https://lists.freebsd.org/pipermail/freebsd-arch/2019-August/019674.html)
Differential Revision: https://reviews.freebsd.org/D21942