Enable build on powerpc64. (It has been tested to build correctly.)
Right now powerpc* are still GCC-based, so openmp functionality is
still not in the base system by default.
Looking at the errorlog results might cause you to think this change
would unblock 308 builds, in reality, all but 28 of them are otherwise
blocked on lang/R, where powerpc64 is not in ONLY_FOR_ARCHS.
Approved by: portmgr (tier-2 blanket)