dtrace: implement riscv dtrace_getustackdepth()

Pretty trivial following other implementations. The existing
dtrace_getustack_common() does most of the work.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38303

(cherry picked from commit c6943b44f7d73176acd2e36e0615e1b2ded02c0a)