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/ghost-param.cton

@@ -9,7 +9,7 @@ isa intel haswell
 ;
 ; Test case by binaryen fuzzer!
 
-function %pr215(i64 vmctx [%rdi]) native {
+function %pr215(i64 vmctx [%rdi]) system_v {
 ebb0(v0: i64):
     v10 = iconst.i64 0
     v1 = bitcast.f64 v10