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>
diff --git a/Makefile b/Makefile
index d48f52f..11efc41 100644
--- a/Makefile
+++ b/Makefile
@@ -761,10 +761,10 @@
 # change __FILE__ to the relative path from the srctree
 KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 
-KBUILD_CFLAGS	+= -g
+KBUILD_CFLAGS	+= -gdwarf-4
 # $(KBUILD_AFLAGS) sets -g, which causes gcc to pass a suitable -g<format>
 # option to the assembler.
-KBUILD_AFLAGS	+= -g
+KBUILD_AFLAGS	+= -gdwarf-4
 
 # Report stack usage if supported
 # ARC tools based on GCC 7.1 has an issue with stack usage