Use the right type for 64-bit coprocessor registers.

The use of "int" here caused the compiler to believe that it needs to
insert a "sll $n, $n, 0" to sign extend as part of the implicit cast
to uint64_t.

Submitted by: Nathaniel Filardo <nwf20@cl.cam.ac.uk>
Reviewed by: brooks, arichardson
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24457