arm64: add definition for IS_SSTEP_TRAP()

arm64 has a distinct exception code for single-step, so we can use this
to detect when an unexpected SS trap is encountered, or when an expected
one is not. See db_stop_at_pc().

Reviewed by: markj, jhb
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e152c882738ffe26a2f52e420e2a92c014943207)