This change makes each unwind step inspect the instruction at the
return address and, if needed, read r2 from its saved location and
modify the context appropriately.
One point that (maybe) could be improved is to try to find if the
binary being unwound is ELFv1 or ELFv2 at run time, instead of at
compile time, or, find out the correct save location of r2 in some
Reported by Bug 41050