scripts: add ARM support to decodecode

This patch adds support for decoding ARM oopses to scripts/decodecode.
The following things are handled:

 - ARCH and CROSS_COMPILE environment variables are respected.

 - The Code: in x86 oopses is in bytes, while it is in either words (4
   bytes) or halfwords for ARM.

 - Some versions of ARM objdump refuse to disassemble instructions
   generated by literal constants (".word 0x...").  The workaround is to
   strip the object file first.

 - The faulting instruction is marked (liked so) in ARM, but <like so>
   in x86.

 - ARM mnemonics may include characters such as [] which need to be
   escaped before being passed to sed for the "<- trapping instruction"
   substitution.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Michal Marek <mmarek@suse.cz>