[Support][PPC] Fix bot failures due to cd53ded5

Commit https://reviews.llvm.org/rGcd53ded557c3 attempts to fix the
computation in computeHostNumPhysicalCores() to respect Affinity.
However, the GLIBC wrapper of the affinity system call fails with
a default size of cpu_set_t on systems that have more than 1024 CPUs.
This just fixes the computation on such large machines.