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.

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