Fix v8 landmines script.

Currently, some builders are in a clobber-landmine loop.

Clobber deletes the .landmines tracker (sigh). Difficult to
know if we are right after a clobber or if it is first-time
landmines deployment. Also, a landmine-triggered clobber
right after a clobber is not possible. Different clobber
methods for msvs, xcode and make all have different
blacklists of files that are not deleted.

After the branch point, all v8 branch builders have to be
manually clobbered, because the appearance of the
landmines.py script and the first landmine request are in
the same branch CL.

TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/457063002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23012 ce2b1a6d-e550-0410-aec6-3dcde31c8c00