1. If D8_PATH isn't specified, first try to locate 'd8' shell in path,
   and if not found, fallback to the one in tools_path/..
2. Add '--nm=<nm_exec>' parameter to specify 'nm' executable to use.

Review URL: http://codereview.chromium.org/132021


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2225 ce2b1a6d-e550-0410-aec6-3dcde31c8c00