arm64: Make KMSAN aware of exceptions

• Call kmsan_intr_enter() when an exception occurs. This ensures that code running in the exception context does not clobber thread-local KMSAN state.
• Ensure that stack memory containing trap frames is treated as initialized.

Co-authored-by: Alexander Stetsenko <alex.stetsenko@klarasystems.com>
Reviewed by: imp
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D43155