arm64: Disable kernel superpage promotion when KMSAN is configured

The break-before-make operation required to promote or demote a
superpage leaves a window where the KMSAN runtime can trigger a fatal
data abort. More specifically, the code in pmap_update_entry() which
executes after ATTR_DESCR_VALID is cleared may implicitly attempt to
access KMSAN context via curthread, but we may be promoting or demoting
a 2MB page containing the curthread structure.

Reviewed by: imp
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D43158