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
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28942