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/legalize-libcall.cton

@@ -9,7 +9,7 @@ ebb0(v0: f32):
     v1 = floor v0
     return v1
 }
-; check: function %floor(f32 [%xmm0]) -> f32 [%xmm0] native {
-; check: sig0 = (f32) -> f32 native
+; check: function %floor(f32 [%xmm0]) -> f32 [%xmm0] system_v {
+; check: sig0 = (f32) -> f32 system_v
 ; check: fn0 = sig0 %FloorF32
 ; check: v1 = call fn0(v0)