Improve plugin error handling

Improve plugin error handling

ld/

	PR ld/14904
	* ldmain.c (main): Don't check plugin_load_plugins return.

	* lexsup.c (parse_args): Don't check plugin_opt_plugin return.

	* plugin.c (dlerror): New.  Defined if HAVE_DLFCN_H isn't
	defined.
	(plugin_opt_plugin): Change return type to void.  Stop on
	dlopen error and report error with dlerror ().
	(plugin_load_plugins): Change return type to void.  Stop on
	dlsym error and report error with dlerror ().  Don't use
	set_plugin_error.
	(plugin_call_cleanup): Issue an error for each plugin.

	* plugin.h (plugin_opt_plugin): Change return type to void.
	(plugin_load_plugins): Likewise.

ld/testsuite/

	PR ld/14904
	* ld-plugin/plugin-2.d: Update expected error message.
	* ld-plugin/plugin-4.d: Likewise.