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/ir/extfunc.rs

@@ -6,7 +6,7 @@
 //! This module declares the data types used to represent external functions and call signatures.
 
 use ir::{Type, FunctionName, SigRef, ArgumentLoc};
-use isa::RegInfo;
+use isa::{RegInfo, RegUnit};
 use std::cmp;
 use std::fmt;
 use std::str::FromStr;
@@ -133,6 +133,16 @@ impl ArgumentType {
         }
     }
 
+    /// Create an argument type for a special-purpose register.
+    pub fn special_reg(vt: Type, purpose: ArgumentPurpose, regunit: RegUnit) -> ArgumentType {
+        ArgumentType {
+            value_type: vt,
+            extension: ArgumentExtension::None,
+            purpose: purpose,
+            location: ArgumentLoc::Reg(regunit),
+        }
+    }
+
     /// Return an object that can display `self` with correct register names.
     pub fn display<'a, R: Into<Option<&'a RegInfo>>>(&'a self, regs: R) -> DisplayArgumentType<'a> {
         DisplayArgumentType(self, regs.into())