Fix KGDB backtrace on ARM

Modify trapframe decoding to properly analyze trapframe.

Provide method for fixup_pc. It happens, that in some kernel
functions, the GDB stack frame decoder cannot determine both
func name and frame size. This is because these functions
either contain invalid instruction, or their format does
not match standard schema. Detect that scenarios and move
PC accordingly to jump into known function schema, which
GDB is able to parse.

Obtained from: Semihalf
Sponsored by: Juniper Networks
Reviewed by: kib, zbb
Differential Revision: https://reviews.freebsd.org/D5976