Avoid wrong unwind info in clone child

[Roland McGrath]

> The unwind info attached to the clone function is usually wrong for the
> child.  This changes the ppc(64) implementation to terminate the FDE
> before the syscall, similar to the i386 implementation.

That looks reasonable.  I put it in.  It would be nice if we had a test for
this, though it might be hard to put together in machine-independent C.


Thanks,
Roland