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>