mips/ddb: fix MIPS backtrace truncation and MIPS32 register printing.

• Cast 32-bit register values to uintmax_t for use with %jx.
• Add special-case return address handling for MipsKernGenException to avoid early termination of stack walking in the exception handler stack frame.

Submitted by: Michael Zhilin <mizhka@gmail.com>
Reviewed by: ray
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D6907