riscv: Treat pointer register as an input in fpe_store

The contents of the memory is an output, but the pointer to that memory
is an input. This was correct in the original version of D45697, but
when adding appropriate clobbers, the pointer operand was incorrectly
switched to an output rather than left an input for fpe_store.

Approved by: re (cperciva)
Reviewed by: jrtc27
Obtained from: CheriBSD
Fixes: 44d4ee7f3dad ("riscv: add FPE code.")
MFC after: 1 day
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D53441

(cherry picked from commit 91f66f9b377e09f59f481e653d8f64120a183806)
(cherry picked from commit 0b5e4af65d0d3dfaa318b8e6adaaa4297f94af17)