PPC: Fix lswx bounds checks

The lswx instruction checks whether the desired string actually fits
into all defined registers. Unfortunately it does the calculation wrong,
resulting in illegal instruction traps for loads that really should fit.

Fix it up, making Mac OS happier.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target-ppc/mem_helper.c

@@ -100,8 +100,9 @@ void helper_lswx(CPUPPCState *env, target_ulong addr, uint32_t reg,
                  uint32_t ra, uint32_t rb)
 {
     if (likely(xer_bc != 0)) {
-        if (unlikely((ra != 0 && reg < ra && (reg + xer_bc) > ra) ||
-                     (reg < rb && (reg + xer_bc) > rb))) {
+        int num_used_regs = (xer_bc + 3) / 4;
+        if (unlikely((ra != 0 && reg < ra && (reg + num_used_regs) > ra) ||
+                     (reg < rb && (reg + num_used_regs) > rb))) {
             helper_raise_exception_err(env, POWERPC_EXCP_PROGRAM,
                                        POWERPC_EXCP_INVAL |
                                        POWERPC_EXCP_INVAL_LSWX);