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.

cranelift/filetests/parser/keywords.cton

@@ -2,4 +2,4 @@ test cat
 
 ; 'function' is not a keyword, and can be used as the name of a function too.
 function %function() {}
-; check: function %function() native
+; check: function %function() system_v