target/ppc: fix cpu_ov setting for 32-bit

A bug was introduced in following commit:

    dc0ad84 target/ppc: update overflow flags for add/sub

As for 32-bit ppc target extracting bit 63 for overflow is not correct.
Made it dependent on TARGET_LOG_BITS. This had broken booting MacOS
9.2.1 image

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
