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/licm/complex.cton

@@ -1,6 +1,6 @@
 test licm
 
-function %complex(i32) -> i32 native {
+function %complex(i32) -> i32 system_v {
 ebb0(v0: i32):
     jump ebb1(v0)