fix ia64 longjmp() to work from alternate signal-stack

[Ulrich Drepper]

What I don't understand is why you have a separate longjmp
implementation.  It seems identical to the generic version.  And why is
the new setjmp split out in two C files.  They should be merged.
Finally, since the code is only used in nptl there is no need to move
the code to libc.so.

To facilitate using the code I've added some changes to define a symbol
__libc_unwind_longjmp.  For all archs it currently just points to
__libc_longjmp.  For ia64 you should remove the macro definition and
instead add a declaration.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖