target-sparc: Cleanup cpu_src[12] allocation

Now that get_temp_tl is used for get_src[12], we don't need to
pre-allocate these temporaries.

Fallout from this is moving some assignments around cas/casx to
avoid uninitialized variable warnings.

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