On powerpc, physical_package_id may not be available. Currently, this
causes openmp to fall back to flat topology and various affinity tests
fail.
Fix this by parsing core_siblings_list to deterimine which cpus belong
to the same socket. This matches what the testing code does. The code to
parse the CPU list format thankfully already exists.
Fixes https://github.com/llvm/llvm-project/issues/111809.
4722c6b87c