um: enable UBSAN

We can select ARCH_HAS_UBSAN, it works just fine. It had been
enabled and we even used it, but then commit 890a6481
("ubsan: Restore dependency on ARCH_HAS_UBSAN") (correctly)
disabled it again, enable ARCH_HAS_UBSAN to get it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20240701220034.995eb04d656d.Ia29fe091b207fe66b5e26298c1e427ebcf131642@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>