target-ppc: Fix add and subf carry generation in narrow mode

The set of computations used in b5a73f8d8a57e940f9bbeb399a9e47897522ee9a
are only valid if the current word size == target_long size.  This failed
to take ppc64 in 32-bit (narrow) mode into account.

Add a NARROW_MODE macro to avoid conditional compilation.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
