Use proper clang plugin name in GN build.

This uses the proper name of the Clang plugin on Linux (previously it was hardcoded to the Mac-style name).

This also fixes the path naming so the clang plugin is correct no matter where the output directory is.

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/121423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242573 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242573