target-sparc: address_mask(), asi_address_mask() are TARGET_SPARC64 only

The address_mask() and asi_address_mask() functions are only used in
TARGET_SPARC64 configs, so guard with ifdefs to avoid warnings about
unused functions in 32-bit builds.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
