A bug was introduced in following commit:

    dc0ad844 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: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: NNikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Tested-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>