Enable OpenMP for powerpc64
Summary: When powerpc64 switches to LLVM, use this patch to enable
OpenMP as well. OpenMP on PPC is only for 64-bits, so don't make a
32-bit libomp. A change to openmp files is necesssary (under review on
https://reviews.llvm.org/D67190), because it determines ELF format
version based on endianness, which is incorrect.
Reviewed by: alfredo.junior_eldorado.org.br, manpages
Differential Revision: https://reviews.freebsd.org/D21532