`rts_setInCallCapability` sets the thread affinity as well as pins the
numa node. We should also have the ability to set the numa node without
setting the capability affinity. `rts_pinNumaNodeForCapability` function
is added and exported via `RtsAPI.h`.

Previous callers of `rts_setInCallCapability` should now also call
`rts_pinNumaNodeForCapability` to get the same effect as before.

Test Plan:
  ./validate

Reviewers: austin, simonmar, bgamari

Reviewed By: simonmar, bgamari

Subscribers: thomie, niteria

Differential Revision: https://phabricator.haskell.org/D2637

GHC Trac Issues: #12764