Skip LTO tests for --disable-plugin

Don't run LTO tests if compiler is configured with --disable-plugin.

	PR ld/20436
	* testsuite/lib/ld-lib.exp (check_gcc_plugin_enabled): New
	proc.
	(check_lto_available): Return 0 if check_gcc_plugin_enabled
	returns 0.
	(check_lto_fat_available): Likewise.
	(check_lto_shared_available): Likewise.