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/spill.cton

@@ -93,7 +93,7 @@ ebb0(v0: i32):
 
 ; The same value used as indirect callee and argument.
 function %doubleuse_icall1(i32) {
-    sig0 = (i32) native
+    sig0 = (i32) system_v
 ebb0(v0: i32):
     ; not:copy
     call_indirect sig0, v0(v0)
@@ -102,7 +102,7 @@ ebb0(v0: i32):
 
 ; The same value used as indirect callee and two arguments.
 function %doubleuse_icall2(i32) {
-    sig0 = (i32, i32) native
+    sig0 = (i32, i32) system_v
 ebb0(v0: i32):
     ; check: $(c=$V) = copy v0
     call_indirect sig0, v0(v0, v0)