minor tweak to MicroLock slow path

minor tweak to MicroLock slow path

Summary:This diff uses the x86 "pause" instruction or its equivalent when
spinning in the MicroLock slow loop.  This gives a hint to the processor
that it should devote more resources to the other execution contexts
sharing the same core.  This diff also removes an mfence on x86 by using
a slightly stronger memory model on the preceding compare_exchange_weak,
and switches to a more portable way of invoking sched_yield().

Reviewed By: dcolascione

Differential Revision: D3018568

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