[PATCH] benchtests: Add pthread_once common-case test.

[Torvald Riegel]

This adds a benchtest for the common-case scenario for pthread_once.

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).  I'm adding this to investigate
whether we still need the x86-custom pthread_once version written in
assembler, or whether we can use the generic version without a
performance loss (both use the same algorithm).

This needs the other patch that adds the include-sources directive to
scripts/bench.pl.

OK?
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