RFR: 8242459: ForceNUMA and only one available NUMA node hits a guarantee

[Kim Barrett]

Please review this small change to avoid failing assertions when
-XX:+ForceNUMA is used to override the decision to disable UseNUMA
when there is only one NUMA node being used.  There were a couple of
places that assumed that if UseNUMA is true then there must be more
than one NUMA node, but that isn't so.

CR:
https://bugs.openjdk.java.net/browse/JDK-8242459

Webrev:
https://cr.openjdk.java.net/~kbarrett/8242459/open.00/

Testing:
mach5 tier1

Locally (linux-x64) manually ran the provided test case, verifying
failure before changes and working after.