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/parser/instruction_encoding.cton

@@ -13,7 +13,7 @@ ebb1(v0: i32 [%x8], v1: i32):
 @55 v9 = iadd v8, v7
 @a5 [Iret#5] return v0, v8
 }
-; sameln: function %foo(i32, i32) native {
+; sameln: function %foo(i32, i32) system_v {
 ; nextln: ebb1(v0: i32 [%x8], v1: i32):
 ; nextln:     [-,-]$WS v2 = iadd v0, v1
 ; nextln:     [-]$WS trap heap_oob