To make this work according to the OpenMP spec, the runtime needs
information about the hardware topology. On Linux the default way
is to parse /proc/cpuinfo which contains this information for x86
machines but (at least) not for Power architectures.
Fortunately, there is a different code path which is able to get
that data from sysfs. The needed patch has landed in 2006 for
Linux 2.6.16 which is safe to assume nowadays (even RHEL 5 had
a kernel version derived from 2.6.18, and we are now at RHEL 7!).
I thought about switching the default for x86 as well because virtually every Linux system should support these sysfs nodes nowadays, but in the end decided to keep the current code path...