asset: do not use 'assets' directory for iOS

The iOS implementation was just copied from the desktop version, and had
and extra directory 'assets'. Also, files had an incorrect assumption that Arm Darwin should be an iOS. This change addresses these issues.

Fixes golang/go#37177

Change-Id: I8e4de5337fbbe0b7697a8d519c8bded783399244
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/616655
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Hyang-Ah Hana Kim <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: hajimehoshi

asset/asset_darwin_armx.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (linux && !android) || (darwin && !arm && !arm64) || windows
+//go:build (linux && !android) || (darwin && !ios) || windows
 
 package asset
 

asset/asset_desktop.go

@@ -0,0 +1,13 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package asset
+
+import (
+	"os"
+)
+
+func openAsset(name string) (File, error) {
+	return os.Open(name)
+}