Use -g instead of -gstabs in AFLAGS_DEBUG

Use -g instead of -gstabs in AFLAGS_DEBUG
Patch by Haavard Skinnemoen, 30 Aug 2006

In config.mk, -Wa,-gstabs is unconditionally appended to AFLAGS no
matter what the target's preferred debugging format is. This patch
simply replaces -gstabs with -g, so that the default debugging format
for the architecture is used.