arch: use the newly introduced target_bits_64

Convert MIPS and Sparc over to using the new target_bits_64 variable.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

reporter_name="Alexander BIGGA"

reporter_url="http://sourceware.org/ml/crossgcc/2008-07/msg00000.html"

reporter_comment=""