Optimize RISC-V copyin(9)/copyout(9) routines.

The existing copyin(9) and copyout(9) routines on RISC-V perform only a
simple byte-by-byte copy. Improve their performance by performing
word-sized copies where possible.

Submitted by: Mitchell Horne <mhorne063@gmail.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18851