Make stack_save*() more robust on MIPS.

• Validate any stack addresses read from against td_kstack before reading. If an unwind operation would attempt to read outside the bounds of td_kstack, abort the unwind instead.

• For stack_save_td(), don't use the PC and SP from the current thread, instead read the PC and SP from pcb_context[].

• For stack_save(), use the current PC and SP of the current thread, not the values from pcb_regs (the horribly named td_frame of the outermost trapframe). The result was that stack_trace() never logged _any_ kernel frames but only the frame from the saved userspace registers on entry from the kernel.

• Inline the one use of stack_register_fetch().

• Add a VALID_PC() helper macro and simplify types to remove excessive casts in stack_capture().

• Fix stack_capture() to work on compilers written in this century. Don't treat function epilogues as function prologues by skipping additions to SP when searching for a function start.

• Add some comments to stack_capture() and fix some style bugs.

Reviewed by: arichardson
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27358