tags: Use $SRCARCH

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

Caused by commit f81b1be4 (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>