gdb/testsuite: add .debug_rnglists tests

gdb/testsuite: add .debug_rnglists tests

Add tests for the various issues fixed in the previous patches.

Add a new "rnglists" procedure to the DWARF assembler, to allow
generating .debug_rnglists sections.  A trivial change is required to
support the DWARF 5 CU header layout.

gdb/testsuite/ChangeLog:

	PR gdb/26813
	* lib/dwarf.exp (_handle_DW_FORM): Handle DW_FORM_rnglistx.
	(cu): Generate header for DWARF 5.
	(rnglists): New proc.
	* gdb.dwarf2/rnglists-multiple-cus.exp: New.
	* gdb.dwarf2/rnglists-sec-offset.exp: New.

Change-Id: I5b297e59c370c60cf671dec19796a6c3b9a9f632