If a plugin has been loaded already, we should warn and return, instead
of adding it on the plugin list.
PR ld/20321
* plugin.c (plugin_opt_plugin): Warn and return if plugin has
been loaded already.
* testsuite/ld-plugin/lto.exp: Run PR ld/20321 test.
* testsuite/ld-plugin/pr20321.c: New file.
