Make sure <arch-gcc> come with consistent content

Expand hackery in post-install target to
make sure that we always end up with same set of
$GCC_TARGET-prefixed files no matter if port is
built for cross target or for native target to be
used as external toolchain.

Take over the maintainership.

Reviewed by: emaste, imp, bapt
Approved by: bapt
Differential Revision: https://reviews.freebsd.org/D10537