folly | Add stub for ThreadId when building with emscripten.

Summary: No thread id available built-in in emscripten, even though it has partial pthread support. Stub it out.

Reviewed By: boguscoder

Differential Revision: D31754569

fbshipit-source-id: 29113ba31cf6b65e7d5c52455fd99ee1e659c041

folly/system/ThreadId.cpp

@@ -52,6 +52,8 @@ uint64_t getOSThreadID() {
   return uint64_t(tid);
 #elif defined(__XROS__)
   return uint64_t(xr_execution_get_id());
+#elif defined(__EMSCRIPTEN__)
+  return 0;
 #else
   return uint64_t(syscall(FOLLY_SYS_gettid));
 #endif