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/x64/mod.rs

@@ -51,7 +51,7 @@ impl X64Backend {
         // This performs lowering to VCode, register-allocates the code, computes
         // block layout and finalizes branches. The result is ready for binary emission.
         let emit_info = EmitInfo::new(flags.clone(), self.x64_flags.clone());
-        let abi = Box::new(abi::X64ABICallee::new(&func, self)?);
+        let abi = Box::new(abi::X64ABICallee::new(&func, self, &self.x64_flags)?);
         compile::compile::<Self>(&func, self, abi, &self.reg_env, emit_info)
     }
 }