Mk/bsd.gcc.mk: Remove support for USE_GCC=X+

At this point most ports that employ USE_GCC have the USE_GCC=yes
form, some have USE_GCC=X (where X is an older version of GCC than
the current default), and none is left with USE_GCC=X+.

To reduce complexity and since we are actively tracking upstream GCC
with our default version, remove support for the USE_GCC=X+ form.

This also derisks Mk/Uses/fortran.mk which aligns with USE_GCC after
commit 4191c71fbd229e5a96382bc6fa271a1ce5668b0f. [1]

PR: 266196 [1]