HACK: Fix the backstrace for Linux/x86-64.

[Lu, Hongjiu]

>        xorq %rbp, %rbp
>
>The ABI doesn't suggest that at all (at least I can't find it) and
>usage of %rbp as a frame pointer is optional, and certainly not the
>default.  This was probably just copied from the i386 code, and should
>be removed.  Then your "hack" doesn't work anymore.
>

FYI, the x86-64 ABI is based on the i386 ABI, which does suggest set
frame pointer to zero. I believe glibc is correct in doing so. Again if
you have a different patch, I will give it a try.


H.J.