kbuild: try readelf first in gen_symversions

deepin inclusion
category: performance

Instead of nm, readelf is more faster, and significantly improve build speed
when enable CONFIG_GENKSYMS.

Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: opsiff

scripts/Makefile.build

@@ -171,7 +171,7 @@ ifdef CONFIG_MODVERSIONS
 #   be compiled and linked to the kernel and/or modules.
 
 gen_symversions =								\
-	if $(NM) $@ 2>/dev/null | grep -q ' __export_symbol_'; then		\
+	if $(READELF) -sW $@ 2>/dev/null | grep -q '__export_symbol_'; then		\
 		$(call cmd_gensymtypes_$(1),$(KBUILD_SYMTYPES),$(@:.o=.symtypes)) \
 			>> $(dot-target).cmd;					\
 	fi