testsuite: Add { dg-lto-do incr-link } option

Similar to { dg-lto-do ar-link }, which allows testing the combination of
multiple LTO objects into an archive before linking, this new directive
allows testing the incremental linking modes of the LTO front end.

gcc/ChangeLog:

	* doc/sourcebuild.texi: Document ar-link and incr-link options to
	dg-lto-do directive.

gcc/testsuite/ChangeLog:

	* gcc.dg/lto/README: Update to document ar-link and incr-link.
	* lib/lto.exp: Support incr-link directive throughout.