[tautschnig]

pthread_rwlock_unlock asserted that the lock byte is 1, which is the
value set by the read-lock functions. pthread_rwlock_wrlock and
pthread_rwlock_trywrlock set it to 2, so unlocking a write lock always
reported a spurious "must hold lock upon unlock" failure. Accept both
values.

The previously KNOWNBUG regression test is now CORE.

Fixes #9338

Found by Jeroen Robben's specification-conformance pipeline (https://reports.robben.io/methodology/).

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