[Bug 239813] Update lang/gcc9, lang/gcc9-devel, lang/gcc8, and lang/gcc8-devel to ELFv2 ABI on powerpc64

[Mark Linimon]

> devel/freebsd-gcc6
> devel/freebsd-gcc6 at aarch64

These two ports are exactly equivalent.

I did not have enough time before the commit to puzzle out a way to
work around that.  I have limited understanding of flavors.

The way it *should* work IMHO is for the former to refuse to build
with a message like "is a meta port -- nothing to build."  This is
used in several other existing masterports.

mcl