tags: Use $SRCARCH

$ make mrproper
$ make tags
  GEN     tags
  find: `arch/x86_64/': No such file or directory

Caused by commit f81b1be (tags: include headers before source files)

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/tags.sh

@@ -5,7 +5,7 @@
 # mode may be any of: tags, TAGS, cscope
 #
 # Uses the following environment variables:
-# ARCH, SUBARCH, srctree, src, obj
+# ARCH, SUBARCH, SRCARCH, srctree, src, obj
 
 if [ "$KBUILD_VERBOSE" = "1" ]; then
 	set -x
@@ -70,7 +70,7 @@ find_sources()
 
 all_sources()
 {
-	find_arch_include_sources ${ARCH} '*.[chS]'
+	find_arch_include_sources ${SRCARCH} '*.[chS]'
 	if [ ! -z "$archinclude" ]; then
 		find_arch_include_sources $archinclude '*.[chS]'
 	fi