riscv locore.S: load constant prior to loop

A very minor micro-optimization; t0 is not clobbered between the loop top and
bottom and there appear to be no other branches to this label.

Submitted by: Nathaniel Filardo <nwf20@cl.cam.ac.uk>
Reviewed by: mhorne
Differential Revision: https://reviews.freebsd.org/D25524