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/wasm/src/code_translator.rs

@@ -1151,7 +1151,6 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
             segment,
             table: table_index,
         } => {
-            // The WebAssembly MVP only supports one table and we assume it here.
             let table = state.get_table(builder.func, *table_index, environ)?;
             let len = state.pop1();
             let src = state.pop1();