[PATCH] Fix synchronization of TPP min/max priorities.

[Torvald Riegel]

This fixes a synchronization bug in nptl/tpp.c and consumers.  The
previous code attempts to do at-least-once initialization using one of
the initialized values as flag for initialization and an
atomic_write_barrier -- but none of the readers had a matching
atomic_read_barrier.  This is not correct on weak-memory-model
architectures, or if the compiler reorders.

The new scheme fixes the synchronization and avoids the acquire barrier
by treating each of the two values as separate at-least-once initialized
values.  See the code comments for details.

No regressions on x86_64.  OK to commit?
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