Modern FreeBSD doesn't come with GCC by default anymore and doesn't even
provide the "gcc" or "g++" falback that OS X does. So there's no point
in keeping the freebsd-clang mkspec in unsupported/ since it's the only
one that works, or keeping the freebsd-g++* ones outside, as they won't
compile.

I'm not removing the GCC mkspecs because you can still install GCC from
the ports tree.

[ChangeLog][FreeBSD] The "freebsd-clang" mkspec is no longer in the
unsupported/ subdir. If you have scripts you use to build Qt, you'll need to
update them to say -platform freebsd-clang or remove the -platform argument.

Change-Id: I7a9e11d7b64a4cc78e24ffff142dfc11d3aabb1e
Reviewed-by: Raphael Kubo da Costa <rakuco@FreeBSD.org>
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>