Codegen: Align representation of stackmap with SpiderMonkey

This commit aligns the representation of stackmaps to be the same
as Spidermonkey's by:
 * Reversing the order of the bitmap from low addresses to high addresses
 * Including incoming stack arguments
 * Excluding outgoing stack arguments

Additionally, some accessor functions were added to allow Spidermonkey
to access the internals of the bitmap.

cranelift/filetests/src/test_binemit.rs

@@ -142,7 +142,10 @@ impl SubTest for TestBinEmit {
             .values()
             .map(|slot| slot.offset.unwrap())
             .min();
-        func.stack_slots.frame_size = min_offset.map(|off| (-off) as u32);
+        func.stack_slots.layout_info = min_offset.map(|off| ir::StackLayoutInfo {
+            frame_size: (-off) as u32,
+            inbound_args_size: 0,
+        });
 
         let opt_level = isa.flags().opt_level();