clang: add support for asmjs arch and Emscripten OS

This only adds support for the arch/OS and doesn't allow anything else in LLVM
for now. There's a corresponding LLVM patch which adds the Triple values.

LLVM patch:
  https://codereview.chromium.org/1024073002/

R=jvoung@chromium.org, azakai@mozilla.com, sunfish@mozilla.com
BUG= https://code.google.com/p/nativeclient/issues/detail?id=4102
TEST= make check-all

Review URL: https://codereview.chromium.org/1022123003
14 files changed