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/native/src/lib.rs

@@ -132,6 +132,14 @@ pub fn builder_with_options(infer_native_flags: bool) -> Result<isa::Builder, &'
         if std::arch::is_aarch64_feature_detected!("lse") {
             isa_builder.enable("has_lse").unwrap();
         }
+
+        if std::arch::is_aarch64_feature_detected!("paca") {
+            isa_builder.enable("has_pauth").unwrap();
+        }
+
+        if cfg!(target_os = "macos") {
+            isa_builder.enable("sign_return_address_with_bkey").unwrap();
+        }
     }
 
     // There is no is_s390x_feature_detected macro yet, so for now