This HEAD commit just makes cast-function-type-strict fatal.
It will stay at the HEAD, and prior commits contain the
actual fixes, organized by the part of the code that the fix
targets. Future changes will be force pushed to have their
fixes come before this HEAD.

This branch cherry-picks patches from daniel@mariadb.org
(in bb-10.5-MDEV-34508-ubsan-errors) which initially
incorporate the flag into the build, as well as fix some of
its findings.