Various fixes for the MIPS DDB stack unwinder.

• Fix kernel stack unwinding end-of-function false-positive
  
  The kernel stack unwinder assumes that any jr $ra indicates the end of the current function. However, modern compilers generate code that contains jr $ra at various places inside the function.

• Handle LLD inter-function padding when looking for the start of a function.

• Use call site for symbol name/offset when unwinding
  
  Currently we use the return address, which will normally just give an output that's off by 8 from the actual call site. However, for tail calls, this is particularly bad, as we end up printing the symbol name for the function that comes after the one that made the call. Instead we should go back two instructions from the return address for the unwound program counter.

Submitted by: arichardson (1, 2), jrtc27 (3)
Reviewed by: arichardson
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27363