Testsuite: add dg-{begin|end}-multiline-output commands

Testsuite: add dg-{begin|end}-multiline-output commands

This patch adds an easy way to write tests for expected multiline
output.  For example we can test carets and underlines for
a particular diagnostic with:

/* { dg-begin-multiline-output "" }
 typedef struct _GMutex GMutex;
                ^~~~~~~
   { dg-end-multiline-output "" } */

multiline.exp is used by prune.exp; hence we need to load it before
prune.exp via *load_gcc_lib* for the testsuites of the various
non-"gcc" support libraries (e.g. boehm-gc).

gcc/testsuite/ChangeLog:
	* lib/multiline.exp: New file.
	* lib/prune.exp: Load multiline.exp.
	(prune_gcc_output): Call into multiline.exp to handle any
	multiline output directives.
	* lib/libgo.exp: Load multiline.exp before prune.exp, using
	load_gcc_lib.

boehm-gc/ChangeLog:
	* testsuite/lib/boehm-gc.exp: Load multiline.exp before
	prune.exp, using load_gcc_lib.

libatomic/ChangeLog:
	* testsuite/lib/libatomic.exp: Load multiline.exp before
	prune.exp, using load_gcc_lib.

libgomp/ChangeLog:
	* testsuite/lib/libgomp.exp: Load multiline.exp before prune.exp,
	using load_gcc_lib.

libitm/ChangeLog:
	* testsuite/lib/libitm.exp: Load multiline.exp before prune.exp,
	using load_gcc_lib.

libvtv/ChangeLog:
	* testsuite/lib/libvtv.exp: Load multiline.exp before prune.exp,
	using load_gcc_lib.

From-SVN: r228655