The port comes with rather old pregenerated GNU autoconf bits which
are unaware of the modern machine architectures, e.g. aarch64 et al.
Previous (even older) version would emit a warning but continue the
configure process; now this is hard error. Additionally, configure
script copies its own config.{guess,sub} files over the ones coming
from our templates. This is easy to untangle, but if done properly,
would require to rebootstrap the whole thing (USES+=autoreconf).
Instead, simply extend the existing *-unknown fallback to treat any
unmatched *-portbld system type similarly (i.e., gracefully accept)
as, prima facie, the build logs remain functionally identical.
While here, whack default optimizations since CFLAGS are respected.
Reported by: fluffy, pkg-fallout
