testsuite: Add scan-ltrans-rtl* for use in dg-final [PR116140]

This extends the scan-ltrans-tree* helpers to create RTL variants.  This
is needed to check the behaviour of an RTL pass under LTO.

gcc/ChangeLog:

	PR libstdc++/116140
	* doc/sourcebuild.texi: Document ltrans-rtl value of kind for
	scan-<kind>-dump*.

gcc/testsuite/ChangeLog:

	PR libstdc++/116140
	* lib/scanltranstree.exp (scan-ltrans-rtl-dump): New.
	(scan-ltrans-rtl-dump-not): New.
	(scan-ltrans-rtl-dump-dem): New.
	(scan-ltrans-rtl-dump-dem-not): New.
	(scan-ltrans-rtl-dump-times): New.