riscv: decode syscall in ddb backtrace
This presents the existing information in a slightly more readable way.
Reviewed by: jrtc27, markj, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36565
(cherry picked from commit 791bfa60e8ac85381d449bae5caa0999950a7039)