Use hyperbarrier by default on all architectures

All architectures except x86_64 used the linear barrier implementation
by default which doesn't give good performance for a larger number
of threads.

Improvements for PARALLEL overhead (EPCC) with this patch on a Power8
system (2 sockets x 10 cores x 8 threads, OMP_PLACES=cores)

 20 threads:  4.55us -> 3.49us
 40 threads:  8.84us -> 4.06us
 80 threads: 19.18us -> 4.74us
160 threads: 54.22us -> 6.73us

Differential Revision: https://reviews.llvm.org/D40358

llvm-svn: 320152