Append link and sret arguments in legalize_signature.

These special-purpose arguments and return values are only relevant for
the function being compiled, so add a `current` flag to
legalize_signature().

- Add the necessary argument values to the entry block to represent
  the special-purpose arguments.
- Propagate the link and sret arguments to return instructions if the
  legalized signature asks for it.

lib/cretonne/src/isa/riscv/mod.rs

@@ -78,9 +78,9 @@ impl TargetIsa for Isa {
                 })
     }
 
-    fn legalize_signature(&self, sig: &mut Signature) {
+    fn legalize_signature(&self, sig: &mut Signature, current: bool) {
         // We can pass in `self.isa_flags` too, if we need it.
-        abi::legalize_signature(sig, &self.shared_flags)
+        abi::legalize_signature(sig, &self.shared_flags, current)
     }
 
     fn emit_inst(&self, func: &Function, inst: Inst, sink: &mut CodeSink) {