Add GET_STACK_USAGE() for arm64.

Its absence meant that GEOM direct dispatch was disabled (the service
routines check the current thread's stack usage to determine whether
to hand off the request to a dedicated thread), and this change is
sufficient to enable direct dispatch by default.

Reviewed by: allanjude
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D15527