Add new headers_check_all target for checking all arches in one go.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

--- a/Makefile
+++ b/Makefile
@@ -936,6 +936,12 @@ headers_install: include/linux/version.h scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.headersinst obj=include
 
+PHONY += headers_check_all
+headers_check_all: headers_install_all
+	$(Q)for arch in $(HDRARCHES); do \
+	 $(MAKE) ARCH=$$arch -f $(srctree)/scripts/Makefile.headersinst obj=include BIASMDIR=-bi-$$arch HDRCHECK=1 ;\
+	 done
+
 PHONY += headers_check
 headers_check: headers_install
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.headersinst obj=include HDRCHECK=1