target-sparc: Remove cpu_tmp0 as a global

Subroutines do their own local temporary management.
Within disas_sparc_insn we limit the existance of the variable
to OP=2 insns, and delay initialization as late as is reasonable
for the specific XOP.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
