Support -plugin-opt=disable-output

LLVM plugin uses --plugin-opt=disable-output to disable output.
all_symbols_read_hook() calls exit() to disable output when
--plugin-opt=disable-output is passed to LLVM plugin.  This patch
registers plugin_cleanup() with atexit(), which removes output file
if --plugin-opt=disable-output is used.

	PR ld/17878
	* configure.ac: Check if atexit exists.
	* config.in: Regenerated.
	* configure: Likewise.
	* plugin.c (plugin_t): Add disable_output.
	(plugin_opt_plugin_arg): Set disable_output for disable-output.
	(plugin_cleanup): New.
	(plugin_load_plugins): Register plugin_cleanup() with atexit().