benchtests: Add pthread_once common-case test.

We have a single thread that runs a no-op initialization once and then
repeatedly runs checks of the initialization (i.e., an acquire load and
conditional jump) in a tight loop.  This gives us, on average, the
best-case latency of pthread_once (the initialization is the
exactly-once slow path, and we're not looking at initialization-related
synchronization overheads in this case).

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