Switch to passing the vmctx hidden argument at the beginning.

This switches to passing the vmctx hidden argument at the beginning of
the argument list, rather than the end.

lib/environ/src/module_environ.rs

@@ -235,8 +235,11 @@ impl<'data> cranelift_wasm::ModuleEnvironment<'data> for ModuleEnvironment<'data
 
 /// Add environment-specific function parameters.
 pub fn translate_signature(mut sig: ir::Signature, pointer_type: ir::Type) -> ir::Signature {
-    sig.params
-        .push(AbiParam::special(pointer_type, ArgumentPurpose::VMContext));
+    // Prepend the vmctx argument.
+    sig.params.insert(
+        0,
+        AbiParam::special(pointer_type, ArgumentPurpose::VMContext),
+    );
     sig
 }