dtrace: handle page faults in riscv dtrace_trap()

We must detect the correct amount to increment sepc, as it may have been
a compressed instruction that triggered the fault.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38299

(cherry picked from commit 06faad1de2c9aadcfd606d5f7121b201dbfbaa9d)