devel/freebsd-gcc6: Retire obsolete port.
Supported FreeBSD versions can no longer be built with GCC 6 and
users wishing to compile FreeBSD with GCC are encouraged to use
devel/freebsd-gcc9 or devel/freebsd-gcc12 instead.
Reviewed by: manu, otis
Differential Revision: https://reviews.freebsd.org/D39218
devel/freebsd-gcc6: fix build on powerpc64 elfv2
Use the same patch that was committed for lang/gcc9 to fix build.
PR: 243933
Reported by: jhb (maintainer)