Fix the COMMENT to tell this is GCC 4.x rather than 3.x. Pointed out
by Simon Barner.
Revert the default debugging format to stabs but offer DWARF-2 as an
option (rather than being the default). AVR-GDB currently understands
stabs much better than DWARF-2.
Fix a bug in patch-zz-atmega256x that caused -fcall-prologues to
generate faulty code. This affected libgcc.a as it uses this option.
This bug has been originally found and fixed by Wang Guohui but I
missed his fix when adding the patch here.