[gdb/testsuite] Add dummy start and end CUs in dwarf assembly

[gdb/testsuite] Add dummy start and end CUs in dwarf assembly

Say one compiles a hello.c:
...
$ gcc -g hello.c
...

On openSUSE Leap 15.2 and Tumbleweed, the CU for hello.c is typically not the
first in .debug_info, nor the last, due to presence of debug information in
objects for sources like:
- ../sysdeps/x86_64/start.S
- init.c
- ../sysdeps/x86_64/crti.S
- elf-init.c
- ../sysdeps/x86_64/crtn.S.

On other systems, say ubuntu 18.04.5, the CU for hello.c is typically the
first and the last in .debug_info.

This difference has caused me to find some errors in the dwarf assembly
using openSUSE, that didn't show up on other platforms.

Force the same situation on other platforms by adding a dummy start
and end CU.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-08-22  Tom de Vries  <tdevries@suse.de>

	PR testsuite/28235
	* lib/dwarf.exp (Dwarf::dummy_cu): New proc.
	(Dwarf::assemble): Add dummy start and end CU.