Add DW_TAG_compile_unit DIE to Dummy CUs

Dummy CUs help detect errors and are very helpful. However, the DWARF
spec seems to indicate the CUs need a DW_TAG_compile_unit in addition to
the header. This patch adds that.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31650

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>
Approved-By: Tom de Vries <tdevries@suse.de>
Tested-By: Tom de Vries <tdevries@suse.de>

gdb/testsuite/lib/dwarf.exp

@@ -2999,6 +2999,7 @@ namespace eval Dwarf {
     proc dummy_cu {} {
 	# Generate a CU with default options and empty body.
 	cu {label dummy_cu} {
+	  compile_unit {}
 	}
 
 	# Generate an .debug_aranges entry for the dummy CU.