Use uint32_t instead of u_long as a storage for breakpoint instruction
to copy. All the platforms breakpoints fits this fine.

This fixes operation on big-endian MIPS64 where we were coping
zeroes instead of real instruction.

Reviewed by: rpaulo
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D8250