Makefile: Enforce DWARF4 output

At this point in time, using DWARF-5 format isn't easy to do by default
with all toolchains that we support.  And relying on the implicit
default can lead to mixing 4 and 5 and then the debug info not being
useful to tools.  For now, enforce using DWARF-4 only.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>