[MIPS] Use generic DWARF_DEBUG

When debugging a kernel compiled by gcc 4.1 with gdb 6.4, gdb could
not show filename, linenumber, etc.  It seems fixed if I used generic
DWARF_DEBUG macro.  Although gcc 3.x seems work without this change,
it would be better to use the generic macro unless there were
something MIPS specific.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>