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/codegen/src/isa/stack.rs

@@ -36,8 +36,9 @@ impl StackRef {
     /// Get a reference to `ss` using the stack pointer as a base.
     pub fn sp(ss: StackSlot, frame: &StackSlots) -> Self {
         let size = frame
-            .frame_size
-            .expect("Stack layout must be computed before referencing stack slots");
+            .layout_info
+            .expect("Stack layout must be computed before referencing stack slots")
+            .frame_size;
         let slot = &frame[ss];
         let offset = if slot.kind == StackSlotKind::OutgoingArg {
             // Outgoing argument slots have offsets relative to our stack pointer.