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/s390x/abi.rs

@@ -73,7 +73,7 @@ use crate::ir::MemFlags;
 use crate::ir::Signature;
 use crate::ir::Type;
 use crate::isa;
-use crate::isa::s390x::inst::*;
+use crate::isa::s390x::{inst::*, settings as s390x_settings};
 use crate::isa::unwind::UnwindInst;
 use crate::machinst::*;
 use crate::machinst::{RealReg, Reg, RegClass, Writable};
@@ -206,9 +206,13 @@ impl Into<MemArg> for StackAMode {
 /// point for the trait; it is never actually instantiated.
 pub struct S390xMachineDeps;
 
+impl IsaFlags for s390x_settings::Flags {}
+
 impl ABIMachineSpec for S390xMachineDeps {
     type I = Inst;
 
+    type F = s390x_settings::Flags;
+
     fn word_bits() -> u32 {
         64
     }
@@ -391,7 +395,7 @@ impl ABIMachineSpec for S390xMachineDeps {
         }
     }
 
-    fn gen_ret(rets: Vec<Reg>) -> Inst {
+    fn gen_ret(_setup_frame: bool, _isa_flags: &s390x_settings::Flags, rets: Vec<Reg>) -> Inst {
         Inst::Ret {
             link: gpr(14),
             rets,