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/global-fixed.cton

@@ -2,7 +2,7 @@ test regalloc
 set is_64bit=1
 isa intel haswell
 
-function %foo() native {
+function %foo() system_v {
 ebb4:
     v3 = iconst.i32 0
     jump ebb3