LoongArch: Makefile: Use "grep -E" instead of "egrep"

The latest version of grep claims the egrep is now obsolete so the build
now contains warnings that look like:
	egrep: warning: egrep is obsolescent; using grep -E

Fix this up by changing the LoongArch Makefile to use "grep -E" instead.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Makefile

@@ -97,7 +97,7 @@ KBUILD_LDFLAGS	+= -m $(ld-emul)
 
 ifdef CONFIG_LOONGARCH
 CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
-	egrep -vw '__GNUC_(MINOR_|PATCHLEVEL_)?_' | \
+	grep -E -vw '__GNUC_(MINOR_|PATCHLEVEL_)?_' | \
 	sed -e "s/^\#define /-D'/" -e "s/ /'='/" -e "s/$$/'/" -e 's/\$$/&&/g')
 endif