Update repair command for Arch Linux (flutter#30428)

The current repair command for Arch Linux is no longer valid because
`lib32-libstdc++5` had been removed from multilib.

Actually, `lib32-gcc-libs` from core just work.
Also see flutter#25035

Author: whhone

packages/flutter_tools/lib/src/base/user_messages.dart

@@ -23,7 +23,7 @@ class UserMessages {
   String get flutterBinariesLinuxRepairCommands =>
       'On Debian/Ubuntu/Mint: sudo apt-get install lib32stdc++6\n'
       'On Fedora: dnf install libstdc++.i686\n'
-      'On Arch: pacman -S lib32-libstdc++5 (you need to enable multilib: https://wiki.archlinux.org/index.php/Official_repositories#multilib)';
+      'On Arch: pacman -S lib32-gcc-libs';
 
   // Messages used in NoIdeValidator
   String get noIdeStatusInfo => 'No supported IDEs installed';