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/regalloc/reload.cton

@@ -5,7 +5,7 @@ isa riscv enable_e
 
 ; Check that we can handle a function return value that got spilled.
 function %spill_return() -> i32 {
-    fn0 = function %foo() -> i32 native
+    fn0 = function %foo() -> i32 system_v
 
 ebb0:
     v0 = call fn0()