Add dependency installation files for x86_64

* The installation script installs the necessary dependencies based on
  host OS version on Ubuntu OS
* clang toolchain BUILD.gn will only use clang-3.5 or above
* Update documentation

Test: gn gen out/Default; ninja -C out/Default
Change-Id: Ibc732ebc68009af25e9a911a724a888e008d45ac