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

[Ulrich Drepper]

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I've applied the patch after minor changes.  There is no need to require
the sigaltstack function being exported from libc.so.  It's a simple
syscall, just inline it.  Plus, even if the function would have needed
to be exported this must have happened only for ia64 and no other arch.

The result seems to work for me but then, I never saw the problem this
is supposed to fix in the first place.

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