[tautschnig]

pthread_rwlock_rdlock used __CPROVER_assume(!lock), which blocks while
any lock is held, including a read lock. Thus a second reader blocked,
and all executions in which two threads hold a read lock at the same
time were excluded. POSIX.1-2017 pthread_rwlock_rdlock(): "A thread
may hold multiple concurrent read locks on rwlock". Only block while a
writer holds the lock (bit 2), and set the reader bit (bit 1).

The previously KNOWNBUG regression test is now CORE.

Fixes #9316

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

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