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/lib.rs

@@ -43,7 +43,7 @@
 //! use cretonne::verifier::verify_function;
 //!
 //! fn main() {
-//!     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));
 //!     let mut fn_builder_ctx = FunctionBuilderContext::<Variable>::new();