[MIPS] die(): Properly declare as non-returning

 This marks the declaration of die() correctly, removing "control reaches
end of non-void function" warnings from non-void functions that die() at
the end.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>