PNaCl doc: mention GCCMM's __atomic_*.

R= dschuff@chromium.org
NOTRY=true
(documentation change)

Review URL: https://codereview.chromium.org/71753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235181 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235181