Rename CallConv::Native to CallConv::SystemV. (#291)

To keep cross-compiling straightforward, Cretonne shouldn't have any
behavior that depends on the host. This renames the "Native" calling
convention to "SystemV", which has a defined meaning for each target,
so that it's clear that the calling convention doesn't change
depending on what host Cretonne is running on.

lib/frontend/src/frontend.rs

@@ -600,7 +600,7 @@ mod tests {
     use Variable;
 
     fn sample_function(lazy_seal: bool) {
-        let mut sig = Signature::new(CallConv::Native);
+        let mut sig = Signature::new(CallConv::SystemV);
         sig.returns.push(AbiParam::new(I32));
         sig.params.push(AbiParam::new(I32));