tsan: optimize test-only barrier

The updated lots_of_threads.c test with 300 threads
started running for too long on machines with low
hardware parallelism (e.g. taskset -c 0-1).
On lots of CPUs it finishes in ~2 secs. But with
taskset -c 0-1 it runs for hundreds of seconds
effectively spinning in the barrier in the sleep loop.

We now have the handy futex API in sanitizer_common.
Use it instead of the passive spin loop.
It makes the test run only faster with taskset -c 0-1,
it runs for ~1.5 secs, while with full parallelism
it still runs for ~2 secs (but consumes less CPU time).

Depends on D107131.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D107132

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