Reftypes part two: add support for stackmaps.

This commit adds support for generating stackmaps at safepoints to the
new backend framework and to the AArch64 backend in particular. It has
been tested to work with SpiderMonkey.

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -1258,6 +1258,10 @@ impl MachInst for Inst {
         15
     }
 
+    fn ref_type_rc(_: &settings::Flags) -> RegClass {
+        RegClass::I64
+    }
+
     type LabelUse = LabelUse;
 }
 
@@ -1273,6 +1277,18 @@ impl MachInstEmit for Inst {
     fn emit(&self, sink: &mut MachBuffer<Inst>, flags: &settings::Flags, state: &mut Self::State) {
         emit::emit(self, sink, flags, state);
     }
+
+    fn pretty_print(&self, mb_rru: Option<&RealRegUniverse>, _: &mut Self::State) -> String {
+        self.show_rru(mb_rru)
+    }
+}
+
+impl MachInstEmitState<Inst> for EmitState {
+    fn new(_: &dyn ABIBody<I = Inst>) -> Self {
+        EmitState {
+            virtual_sp_offset: 0,
+        }
+    }
 }
 
 /// A label-use (internal relocation) in generated code.