[Linux] Add NaCl support

Based on NaCl component work (http://crbug.com/244791) from upstream,
this commit is to add Native Client(aka NaCl) support.
It now can run examples from nacl sdk.
  $ cd <nacl_sdk_dir>/pepper_canary/example
  $ TOOLCHAIN=newlib make serve
  $ xwalk http://localhost:5103

Next steps:
 * Enable PNaCl
 * Enable on Tizen

BUG=XWALK-1338