Mk/bsd.gcc.mk: Never use /usr/bin/gcc

USE_GCC=any was introduced to leverage the old version of GCC 4.2
installed as /usr/bin/gcc on some systems. That has increasingly
not been present any longer (not on i386 and amd64 since 12.x and
optionally 11.x, not even on the ppcdevref system according to
linimon@) and hardly anyone actually has been testing ports in
this scenario.

So, finally stop using /usr/bin/gcc (and /usr/bin/gc++ and
/usr/bin/gcpp) even if present.

This makes USE_GCC=any just another way of spelling USE_GCC=yes
before we finally de-orbit it.

Discussed with: mat, linimon, pkubaj