Improve backtrace formatting.

Show the symbol offset, when available.

Centralized formatting of native stack traces in libcorkscrew.
It's handy for automated tools if all stacks look the same.
Since we already made them all look them same, we might as well
do the formatting in just one place.

Do not strip the Thumb bit on ARM.  This fixes an off-by-one
issue that could happen when resolving a PC that was at the
very beginning of a Thumb function, since the symbol table would
have the Thumb bit set but since we stripped the bit from our
PC, we would be looking for an address one byte before the
one listed in the symbol table.  It's also quite useful to see
whether a given function is executing in Thumb mode just by glancing
at the PC.

Change-Id: Icaa29add85ce0bcafe24d5ce2098e138d809e2ab