Initial back-edge CFI implementation (#3606)

Give the user the option to sign and to authenticate function
return addresses with the operations introduced by the Pointer
Authentication extension to the Arm instruction set architecture.

Copyright (c) 2021, Arm Limited.

cranelift/codegen/src/isa/aarch64/inst/emit_tests.rs

@@ -38,6 +38,25 @@ fn test_aarch64_binemit() {
     //
     //      $ echo "mov x1, x2" | aarch64inst.sh
     insns.push((Inst::Ret { rets: vec![] }, "C0035FD6", "ret"));
+    insns.push((
+        Inst::AuthenticatedRet {
+            key: APIKey::A,
+            is_hint: true,
+            rets: vec![],
+        },
+        "BF2303D5C0035FD6",
+        "autiasp ; ret",
+    ));
+    insns.push((
+        Inst::AuthenticatedRet {
+            key: APIKey::B,
+            is_hint: false,
+            rets: vec![],
+        },
+        "FF0F5FD6",
+        "retab",
+    ));
+    insns.push((Inst::Pacisp { key: APIKey::B }, "7F2303D5", "pacibsp"));
     insns.push((Inst::Nop0, "", "nop-zero-len"));
     insns.push((Inst::Nop4, "1F2003D5", "nop"));
     insns.push((Inst::Csdb, "9F2203D5", "csdb"));