Fix NUMA support on Windows (Trac #15049)

• osNumaNodes now returns the right number of nodes
• thread affinity is now correctly set

TODO: no noticeable performance improvement.
does windows already distribute threads in a NUMA-aware fashion?

Test Plan:

• validate
• local tests on a NUMA machine

Reviewers: bgamari, erikd, simonmar

Reviewed By: bgamari, simonmar

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4607