[IA64] die_if_kernel() can return

arch/ia64/kernel/unaligned.c erroneously marked die_if_kernel()
with a "noreturn" attribute ... which is silly (it returns whenever
the argument regs say that the fault happened in user mode, as one
might expect given the "if_kernel" part of its name!).  Thanks to
Alan and Gareth for pointing this out.

Signed-off-by: Tony Luck <tony.luck@intel.com>