Default debug format for AVR

[Simon Marchi]

Hi,

The default debug format (when using only -g) for the AVR target is
stabs.  Is there a reason for it not being DWARF, and would it be
possible to maybe consider possibly thinking about making it default to
DWARF?  I am asking because the support for stabs in GDB is pretty much
untested and bit-rotting, so I think it would be more useful for
everyone to use DWARF.

Simon