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/verifier/defs_dominates_uses.cton

@@ -2,14 +2,14 @@ test verifier
 
 ; Test verification that uses properly dominate defs.
 
-function %non_dominating(i32) -> i32 native {
+function %non_dominating(i32) -> i32 system_v {
 ebb0(v0: i32):
     v1 = iadd.i32 v2, v0   ; error: uses value from non-dominating
     v2 = iadd.i32 v1, v0
     return v2
 }
 
-function %inst_uses_its_own_values(i32) -> i32 native {
+function %inst_uses_its_own_values(i32) -> i32 system_v {
 ebb0(v0: i32):
     v1 = iadd.i32 v1, v0   ; error: uses value from itself
     return v1