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/domtree/loops.cton

@@ -59,7 +59,7 @@ function %test(i32) {
 ; nextln: ebb5:
 ; nextln: }
 
-function %loop2(i32) native {
+function %loop2(i32) system_v {
     ebb0(v0: i32):
         brz v0, ebb1    ; dominates: ebb1 ebb3 ebb4 ebb5
         jump ebb2       ; dominates: ebb2

cranelift/filetests/domtree/loops2.cton

@@ -43,7 +43,7 @@ function %loop1(i32) {
 ; nextln: ebb9:
 ; nextln: }
 
-function %loop2(i32) native {
+function %loop2(i32) system_v {
     ebb0(v0: i32):
         brz v0, ebb1    ; dominates: ebb1 ebb3 ebb4 ebb5
         jump ebb2       ; dominates: ebb2