Use register_t instead of uintptr_t for register values in backtraces.

This fixes backtraces from DDB in n32 kernels as uintptr_t is only a
uint32_t. In particular, the upper 32-bits of each register value were
treated as the register's value breaking both the output of register
values, but also the values of 'ra' and 'sp' required to walk up to the
previous frame.

Sponsored by: DARPA / AFRL