It turns out that the hang that was fixed in
commit 43d3dad1
for the SRW_LOCK_DUMMY implementation is also possible in the futex
implementation. We have observed hangs of ssux_lock_low::u_unlock()
on Windows where the undesirable value is rw_lock::UPDATER, in the
test mariabackup.xb_compressed_encrypted.

The exact sequence of events to the hang is not known, but
it seems that u_unlock() had better always wake up one thread.
Possibly, the case involves multiple blocked u_unlock().

On a busy server, the hang might be 'rescued' by a subsequent
lock acquisition and release that is executed by another thread.

rw_lock::update_unlock(): Change the return type to void.

ssux_lock_low::u_unlock(): Always invoke readers_wake() [sic],
to wake up any pending update_lock() or write_lock().
On futex implementation, this will wake up all waiters.
On SRW_LOCK_DUMMY, writer_wake() and readers_wake() do the same
thing: wake up one write_lock(), or all update_lock() waiters.