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="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/"

 reporter_comment="Toolchain for the Raspberry Pi, with hard-float."