[Linux] Add basic NaCl support

This is initial work to add NaCl support for Crosswalk. It now can run
Hello world from nacl sdk.
  $ cd <nacl_sdk_dir>/pepper_canary/getting_started
  $ TOOLCHAIN=newlib make serve
  $ xwalk http://localhost:5103/part2

Next steps:
 * Enable PNaCl
 * Run all examples from nacl sdk
 * Enable on Tizen

BUG=XWALK-1338