Summary:
The problem boils down to global variables: in particular gc_threads[],
which was being modified by a subsequent GC before the previous GC had
finished with it.  The fix is to not use global variables.

This was causing setnumcapabilities001 to fail (again!).  It's an old
bug though.

Test Plan:
Ran setnumcapabilities001 in a loop for a couple of hours.  Before this
patch it had been failing after a few minutes.  Not a very scientific
test, but it's the best I have.

Reviewers: bgamari, austin, fryguybob, niteria, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2654