kbuild: fix genksyms build error

genksyms needs to know when a symbol must have a "_" prefex as is
true for a few architectures.
Pass $(ARCH) as commandline argument and hardcode what architectures that
needs this info.
Previous attemt to take it from elfconfig.h was br0ken since elfconfig.h
is a generated file.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 7afe3e7..19ef2bc 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -166,7 +166,7 @@
 cmd_modversions =							\
 	if $(OBJDUMP) -h $(@D)/.tmp_$(@F) | grep -q __ksymtab; then	\
 		$(CPP) -D__GENKSYMS__ $(c_flags) $<			\
-		| $(GENKSYMS)						\
+		| $(GENKSYMS) -a $(ARCH)				\
 		> $(@D)/.tmp_$(@F:.o=.ver);				\
 									\
 		$(LD) $(LDFLAGS) -r -o $@ $(@D)/.tmp_$(@F) 		\