Further refine MIPS stack traces across trapframes.

Use the trapframe unwinder recently added for kernel stack overflow
panics for frames crossing MipsKernGenException and MipsKernIntr.
This provides more reliably unwinding across nested interrupts and
exceptions in the kernel.

While here, dump the value of the CAUSE and BADVADDR registers when
crossing a trapframe.

Submitted by: rwatson (original version)
Obtained from: CheriBSD
Sponsored by: DARPA / AFRL