ARM backtrace

Hello folks,
http://www.rockbox.org/tracker/task/12302 adds backtrace log to the
panic screen on ARMs. This is simple port of
http://www.mcternan.me.uk/ArmStackUnwinding/. The approach to take
this backtrace is not perfect and has limitations BUT it doesn't need
any special symbols or something to unwind stack frames. I am aware
that the code itself would need some polish (unneeded typedefs, dos
line ends etc.) but first I am interested if we want something like
this at all. I played a bit with this inserting asm snippet causing
data abort in various places and produced backtrace looked reasonable
- still more tests needed.

wo