Add RETGUARD implementation for powerpc and powerpc64.

Index: lib/Target/PowerPC/PPCFrameLowering.cpp
--- lib/Target/PowerPC/PPCFrameLowering.cpp.orig
+++ lib/Target/PowerPC/PPCFrameLowering.cpp
@@ -15,6 +15,7 @@
 #include "PPCInstrBuilder.h"
 #include "PPCInstrInfo.h"
 #include "PPCMachineFunctionInfo.h"
+#include "PPCReturnProtectorLowering.h"
 #include "PPCSubtarget.h"
 #include "PPCTargetMachine.h"
 #include "llvm/ADT/Statistic.h"
@@ -2696,4 +2697,8 @@ bool PPCFrameLowering::enableShrinkWrapping(const Mach
   if (MF.getInfo<PPCFunctionInfo>()->shrinkWrapDisabled())
     return false;
   return !MF.getSubtarget<PPCSubtarget>().is32BitELFABI();
+}
+
+const ReturnProtectorLowering *PPCFrameLowering::getReturnProtector() const {
+  return &RPL;
 }