target-mips/translate.c: Add ifdef guard around check_mips64()

The function check_mips64() is only used if TARGET_MIPS64 is defined;
add an ifdef guard to its definition to avoid warnings about it being
unused in other configurations.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
