[MIPS] cpu-bugs64.c: GCC 3.3 constraint workaround

Add a workaround to address warnings generated on the "n" constraint by
GCC 3.3 and below.

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