After deprecating the top-file Makefile, "make tags" cannot easily be
used any more. Moving the tags-related rules out to a separate Makefile
allows to continue using it using either "make -f tools/Makefile.tags
tags", or including it in your own Makefile.

R=yangguo@chromium.org
CC=ahaas@chromium.org

Bug: v8:7346
Change-Id: Id2a8186c392fbca4d144b166d598c9e930defa37
Reviewed-on: https://chromium-review.googlesource.com/881018Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50889}