Use the right type for 64-bit coprocessor registers.

[jhb]

Summary

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.

Obtained from: CheriBSD

Test Plan

• built mips tinderbox kernels, also has been in cheribsd