Fix NUMA node for heap when NUMA is not available

Fix NUMA node for heap when NUMA is not available

The recent refactoring of the GCToOSInterface::GetProcessorForHeap has
accidentally changed the NUMA node returned in case NUMA is disabled
(either via the COMPlus_GCNumaAware or due to the fact that there is
just a single NUMA node on the system) and the CPU groups are disabled.
Before that refactoring, the code was incorrectly returning 0 as the
NUMA node when CPU groups were disabled no matter whether NUMA was
enabled or disabled. The refactoring fixed that by returning the
current CPU group number for the case when NUMA was enabled, however
it still returned incorrect value, this time GroupProcNo::NoGroup as
the NUMA node number in case NUMA was disabled.

This change fixes it by returning the current group number in this case.

Commit migrated from https://github.com/dotnet/coreclr/commit/8d93bd9080f47efd87dc5420a6e600f8dc432659