target/ppc: fix cpu_ov setting for 32-bit

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>

target/ppc/translate.c

@@ -818,7 +818,7 @@ static inline void gen_op_arith_compute_ov(DisasContext *ctx, TCGv arg0,
         if (is_isa300(ctx)) {
             tcg_gen_extract_tl(cpu_ov32, cpu_ov, 31, 1);
         }
-        tcg_gen_extract_tl(cpu_ov, cpu_ov, 63, 1);
+        tcg_gen_extract_tl(cpu_ov, cpu_ov, TARGET_LONG_BITS - 1, 1);
     }
     tcg_gen_or_tl(cpu_so, cpu_so, cpu_ov);
 }