Sparc: fix carry flag handling (Solaris bootblk fix)

Sparc: fix carry flag handling (Solaris bootblk fix)

The page 108 of the SPARC Version 8 Architecture Manual describes
that addcc and addxcc shall compute carry flag the same way.
The page 110 claims the same about subcc and subxcc instructions.
This patch fixes carry computation in corner cases and removes redundant code.
The most visible effect of the patch is enabling Solaris boot when using OBP.

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
[blauwirbel@gmail.com: cleaned up formatting]
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>