It would be better to generate ctags file for specified architecture so
this CL adds a script gen-tags.py to generate architecture specific
ctags.

  Usage:
      $ tools/dev/gen-tags.py [<arch>...]

  The example usage for 'x64' is as follows:
      $ tools/dev/gen-tags.py x64

  If no <arch> is given, it generates tags file for all arches:
      $ tools/dev/gen-tags.py

R=yangguo@chromium.org,jochen@chromium.org,jkummerow@chromium.org,clemensh@chromium.org

NOTRY=true

Review-Url: https://codereview.chromium.org/2762903002
Cr-Commit-Position: refs/heads/master@{#44032}