Remove generated makefiles on linux when running gyp_v8

BUG=331475
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/108293006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00