I came up with this patch so I could test the longjmp patches
on x86_64-unknown-linux-gnu and x86-pc-linux-gnu.  It was inspired on 
a 2006 patch by Jan Kratochvil.

I'm not proposing this to go in, as it will brake glibc's where
the pointer mangling is not implemented or is implemented
differently.  Maybe we could get around this 99% of the
times by switching the unmangling algorithm based on glibc's
version, although I don't know how to get at glibc's version.

-- 
Pedro Alves