Remove the __native_client__ system define when using NaCl triple. Emscripten does not implement Google Native Client/Pepper APIs.
Remove __pnacl__ #define as well.
Also remove #define __ELF__ when building using NaCl triple.
Merge pull request #1159 from juj/no_nacl_triple

Remove the __native_client__ system define when using NaCl triple.