Douglas Wilson authored Jan 21, 2018 and Ben Gamari committed Jan 21, 2018

Rename to whitehole_gc_spin, in preparation for adding stats for the
whitehole busy-loop in SMPClosureOps.

Make whitehole_gc_spin volatile, and move it to be defined and
statically initialised in GC.c. This saves some #ifs, and I'm pretty
sure it should be volatile.

Test Plan: ./validate

Reviewers: bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4300