being unfechable as a dependency of another port. Introduce instead
COMPAT4X_ARCH which is the same as ARCH except on amd64 where it is "i386"
and replace relevant occurances of ARCH with it.
This also unbreaks ".if ${ARCH} == amd64" comparison that comes after ARCH
was modified to "i386" before.
While here, avoid installing ldconfig/compat4x for amd64 which only needs
ldconfig32/compat4x.
Unfetchability hinted at by: pointyhat via kris