Add support to automatically search for corresponding architecture ports in merge-to-branch.sh.

BUG=
R=jkummerow@chromium.org, plind44@gmail.com

Review URL: https://codereview.chromium.org/152343011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19163 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/master'