remove some mac multiarch vestiges

ppc/ppc64 and 32-bit x86 have been dead for a while.

consequently, the legacy macx-g++-64 spec was most probably not used.

which in turn meant that NATIVE_64_ARCH was never set (in particular on
windows hosts ...), which means that the android ndk host auto-detection
was effectively broken.

the arch code in mac/default_post.prf was also never triggered, so nuke
it as well.

Change-Id: Ic0775e40b273a22e0a15808cac328e0df33c2155
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>