In the SUX_LOCK_GENERIC implementation, we can remember at most
one pending exclusive lock request. If multiple exclusive lock
requests are pending, the WRITER_WAITING flag will be cleared when
the first waiting writer acquires the exclusive lock.

ssux_lock_low::update_lock(): If WRITER_WAITING is set, wake up
the writer even if the UPDATER flag is set, because the waiting
writer may be in the process of upgrading its U lock to X.

rw_lock::read_unlock(): Also indicate that an X lock waiter must
be woken up if an U lock exists.

This fix may cause unnecessary wake-ups and system calls, but this
is the best that we can do. Ideally we would use the MDEV-25404
idea of a separate 'writer' mutex, but there is no portable way to
request that a non-recursive mutex be created, and InnoDB requires
the ability to transfer buf_block_t::lock ownership to an I/O thread.

To allow problems like this to be caught more reliably in the future,
we add a unit test for srw_mutex, srw_lock, ssux_lock, sux_lock.