target-ppc: Fix narrow-mode add/sub carry output

Broken in b5a73f8d8a57e940f9bbeb399a9e47897522ee9a, the carry itself was
fixed in 79482e5ab38a05ca8869040b0d8b8f451f16ff62.  But we still need to
produce the full 64-bit addition.

Simplify the conditions at the top of the functions for when we need a
new temporary.  Only plain addition is important enough to warrent avoiding
the temporary, and the extra tcg move op that would come with it.

Signed-off-by: Richard Henderson <rth@twiddle.net>