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/docs/example.cton

@@ -1,6 +1,6 @@
 test verifier
 
-function %average(i32, i32) -> f32 native {
+function %average(i32, i32) -> f32 system_v {
     ss1 = explicit_slot 8         ; Stack slot for ``sum``.
 
 ebb1(v1: i32, v2: i32):