Peter Trommler authored Jun 03, 2016 and Ben Gamari committed Jun 04, 2016

Use C compiler builtins for atomic SMP primitives. This saves a lot
of CPP ifdefs.

Add test for atomic xchg:
Test if __sync_lock_test_and_set() builtin stores the second argument.
The gcc manual says the actual value stored is implementation defined.

Test Plan: validate and eyeball generated assembler code

Reviewers: kgardas, simonmar, hvr, bgamari, austin, erikd

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2233