Add retguard for arm64.

Index: lib/Target/AArch64/AArch64InstrInfo.td
--- lib/Target/AArch64/AArch64InstrInfo.td.orig
+++ lib/Target/AArch64/AArch64InstrInfo.td
@@ -721,6 +721,14 @@ def ADDlowTLS
 
 } // isReMaterializable, isCodeGenOnly
 
+//===----------------------------------------------------------------------===//
+// Pseudo instruction used by retguard
+let isCodeGenOnly = 1, hasNoSchedulingInfo = 1 in {
+    let Size = 8 in {
+    def RETGUARD_JMP_TRAP: Pseudo<(outs), (ins GPR64:$reg), []>;
+    }
+}
+
 def : Pat<(AArch64LOADgot tglobaltlsaddr:$addr),
           (LOADgot tglobaltlsaddr:$addr)>;