Patch by Bruce Mitchener. Change all references to EMSCRIPTEN to __EMSCRIPTEN__. If you're not using the PP symbol EMSCRIPTEN, then you should see no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@195136 91177308-0d34-0410-b5e6-96231b3b80d8