MariaDB crash + memory corruption on AArch64

Hi All, This is just a heads-up that we've run into quite a horrible crash + data corruption issue with MariaDB when running sysbench with a large number of threads on a couple of AArch64 platforms.

I believe the problem can be attributed to the code used to release the custom mutexes used by the storage engine.

I have logged a ticket (and attached an emergency fix) here: https://mariadb.atlassian.net/browse/MDEV-6615

Cheers, -- Steve Capper