Only compile clang supporting architectures of amd64, arm, arm64, and
i386 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.
Discussed on: arch@ (https://lists.freebsd.org/pipermail/freebsd-arch/2019-August/019674.html)