gdb/testsuite: add .debug_loclists tests

gdb/testsuite: add .debug_loclists tests

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

Add a new "loclists" procedure to the DWARF assembler, to allow
generating .debug_loclists sections.

gdb/testsuite/ChangeLog:

	PR gdb/26813
	* lib/dwarf.exp (_handle_DW_FORM): Handle DW_FORM_loclistx.
	(loclists): New proc.
	* gdb.dwarf2/loclists-multiple-cus.c: New.
	* gdb.dwarf2/loclists-multiple-cus.exp: New.
	* gdb.dwarf2/loclists-sec-offset.c: New.
	* gdb.dwarf2/loclists-sec-offset.exp: New.

Change-Id: I209bcb2a9482762ae943e518998d1f7761f76928