ARM: libroot: fix setjmp/longjmp implementation.

Just a couple of lines of code, but a head full of pain ;-) Finally
got it right and now KDL can properly recover from invalid accesses.
ARM: kernel: Make KDL more useful on ARM

This adds the -mapcs-frame compiler flag for ARM to have "stable"
stack frames, adds support to the kernel for dumping stack crawls,
and initial support for iframes. There' much more functionality
to unlock in KDL, but this makes debugging already a lot more
comfortable.....