[PATCH v2][malloc] Use relaxed atomics for malloc have_fastchunks

[DJ Delorie]

Results of benchmarks...  Note that dj2 is a synthetic test, so the
slowdown is not surprising, and that all tests are averages of 16 runs
except git-cinnabar-helper which is only two pristine and one patched
(it takes 20 minutes per run and I got impatient ;).  Values are cycles,
lower is better.

Workload	   	    Pristine	    Patched	
389ds			  9,121,687,695	  8,017,021,813	 87.89%
dj2			  7,901,004,232	  8,277,784,940	104.77%
git-cinnabar-helper	100,123,244,269	 90,058,896,622	 89.95%
okular-1		  3,648,656,309	  3,220,751,900	 88.27%
oocalc			  1,053,984,703	  1,009,859,213	 95.81%
qemu-virtio		    781,260,028	    766,458,246	 98.11%
qemu-win7		    655,497,193	    626,270,566	 95.54%
proprietary-2		  2,112,159,165	  1,977,684,058	 93.63%
			
						Mean	 94.25%

Patch looks good to me otherwise, caveat any futher complaints about
concurrency ;-)