target/hppa: Fix unit carry conditions

target/hppa: Fix unit carry conditions

Split do_unit_cond to do_unit_zero_cond to only handle conditions
versus zero.  These are the only ones that are legal for UXOR.
Simplify trans_uxor accordingly.

Rename do_unit to do_unit_addsub, since xor has been split.
Properly compute carry-out bits for add and subtract, mirroring
the code in do_add and do_sub.

Tested-by: Helge Deller <deller@gmx.de>
Reviewed-by: Helge Deller <deller@gmx.de>
Fixes: b2167459ae4 ("target-hppa: Implement basic arithmetic")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>