[gdb/testsuite] Generate .debug_aranges entry for dummy CU

A best practise for DWARF [1] is to generate .debug_aranges entries for CUs
even if they have no address range.

Generate .debug_arange entries for the dummy CUs added by the DWARF assembler.

Tested on x86_64-linux.

[1] http://wiki.dwarfstd.org/index.php?title=Best_Practices

gdb/testsuite/lib/dwarf.exp

@@ -2419,7 +2419,11 @@ namespace eval Dwarf {
     # Emit a dummy CU.
     proc dummy_cu {} {
 	# Generate a CU with default options and empty body.
-	cu {} {
+	cu {label dummy_cu} {
+	}
+
+	# Generate an .debug_aranges entry for the dummy CU.
+	aranges {} dummy_cu {
 	}
     }