Fix computeHostNumPhysicalCores() for Linux on POWER and Linux on Z

ThinLTO is run using a single thread on Linux on Power. The
compute_thread_count() routine calls getHostNumPhysicalCores which
returns -1 by default, and so `MaxThreadCount is set to 1.

unsigned llvm::ThreadPoolStrategy::compute_thread_count() const {
    int MaxThreadCount = UseHyperThreads
          ? computeHostNumHardwareThreads()
          : sys::getHostNumPhysicalCores();
     if (MaxThreadCount <= 0)
        MaxThreadCount = 1;
   …
}
Fix: provide custom implementation of getHostNumPhysicalCores for
Linux on Power and Linux on Z.

Reviewed By: Kai, uweigand

Differential Revision: https://reviews.llvm.org/D84764