For ia64, if the longjmp is crossing a signal-frame, install
the new signal mask by updating the sc_mask member in the
oldest signal-frame.
(Logical change 1.259)
(_longjmp): Ensure that we have at least 4 exception-handling args. (This code
will need updating to make it work on x86, where only 2 exception handling
args are available).
(Logical change 1.59)