gdb/testsuite: use -gdwarf-4 in simavr board

gdb/testsuite: use -gdwarf-4 in simavr board

By default, when using the -g switch, avr-gcc generates stabs debug
information.  I think it would make more sense to test GDB against DWARF
information, because stabs is obsolete by now, and nobody work on it.
So change the simavr board to pass -gdwarf-4 as the debug flag.

The downside is that users are probably more likely to use just -g, so
we don't test GDB the same way as users are likely to use it.  But in
this case, if somebody comes and asks for help with GDB for AVR, I
suggest we encourage them to use -gdwarf-4.

I can't give stats about how that changes test results, because the
testsuite is too broken.  There is an internal error that happens quite
frequently that needs to be investigated:

    /home/simark/src/wt/avr/gdb/trad-frame.h:143: internal-error: LONGEST trad_frame_saved_reg::addr() const: Assertion `m_kind == trad_frame_saved_reg_kind::ADDR' failed.

I sent a question on the gcc mailing list, asking why stabs is the
default:

    https://gcc.gnu.org/pipermail/gcc/2021-April/235309.html

gdb/testsuite/ChangeLog:

	* boards/simavr.exp: Set debug_flags.

Change-Id: I70e471fad3a79ab1d79d13dda8436bb9eb666e0a