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/isa/intel/abi32.cton

@@ -5,14 +5,14 @@ isa intel
 ; regex: V=v\d+
 
 function %f() {
-    sig0 = (i32) -> i32 native
-    ; check: sig0 = (i32 [0]) -> i32 [%rax] native
+    sig0 = (i32) -> i32 system_v
+    ; check: sig0 = (i32 [0]) -> i32 [%rax] system_v
 
-    sig1 = (i64) -> b1 native
-    ; check: sig1 = (i32 [0], i32 [4]) -> b1 [%rax] native
+    sig1 = (i64) -> b1 system_v
+    ; check: sig1 = (i32 [0], i32 [4]) -> b1 [%rax] system_v
 
-    sig2 = (f32, i64) -> f64 native
-    ; check: sig2 = (f32 [0], i32 [4], i32 [8]) -> f64 [%xmm0] native
+    sig2 = (f32, i64) -> f64 system_v
+    ; check: sig2 = (f32 [0], i32 [4], i32 [8]) -> f64 [%xmm0] system_v
 
 ebb0:
     return