testsuite: multiline.exp: implement optional target/xfail selector

testsuite: multiline.exp: implement optional target/xfail selector

gcc/testsuite/ChangeLog:
	* lib/multiline.exp (proc dg-end-multiline-output): Check argument
	count.  If there's a 3rd argument, use dg-process-target on it,
	bailing out, or recording expected failures as "maybe_x".
	(proc handle-multiline-outputs): Extract "maybe_x", and use it
	to convert pass/fail into xpass/xfail.

From-SVN: r264880