(_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).
(longjmp): If we are compiling with GCC, use an alias-attribute to alias it with
_longjmp(). This is more efficient and works around a gcc-3.2/ia64 bug
which causes bad unwind info when a noreturn function is a last call.
(Logical change 1.59)