Use strict atomicModifyIORef' (added in GHC 7.6).

Authored by ezyang.

Description

Use strict atomicModifyIORef' (added in GHC 7.6).

Summary: Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: austin, hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D635