Despite my pathological distrust of spin locks, the number just don't lie.  I've put a small spin in __sp_mut::lock() on std::mutex::try_lock(), which is testing quite well.  In my experience, putting in a yield for every failed iteration is also a major performance booster.  This change makes one of the performance tests I was using (a highly contended one) run about 20 times faster.

llvm-svn: 160967
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