Mk/bsd.gcc.mk: Adjust to the release of GCC 11

Now that GCC 11.1 has been released and lang/gcc11 is in place,
have USE_GCC=11 use that instead of lang/gcc11-devel.

In addition add support for USE_GCC=12 which uses lang/gcc12-devel
(still in early development, not recommended for production use).