As of today, USE_GCC=yes (and USE_GCC=any in most circumstances)
and consequently many of the USES=compiler flavors use the canonical
version of GCC as defined in Mk/bsd.default-versions.mk as well as
the lang/gcc port

With the "new" setup starting with GCC 5 where I have introduced
lang/gcc5-devel for regular snapshots and lang/gcc5 for releases,
and similarly for GCC 6 and onward, we can now leverage lang/gcc5
(and later) for most of the role that lang/gcc used to play -- and
indeed as of today lang/gcc and lang/gcc5 are nearly identical
short of symlinks for gcc, g++, and gfortran binaries that the
former provides.

So now use lang/gcc5 instead of lang/gcc whenever requested via the
USE_GCC framework directly or indirectly.

This is similar to how the python ports work, for example, and it
allows simplifications in Mk/bsd.gcc.mk and Mk/Uses/fortran.mk and
dropping LANG_GCC_IS from Mk/bsd.default-versions.mk. As a next
step lang/gcc is going to become a "hull" essentially only providing
those symlinks and requiring lang/gcc5 (or whatever has been set as
default in Mk/bsd.default-versions.mk).