Implement pthread_cond_broadcast wrapper for lib-pthread (#982)

Implement pthread_cond_broadcast wrapper for lib-pthread
- support pthread_cond_broadcast wrapper for posix/linux-sgx/windows
- update document for building multi-thread wasm app with emcc

doc/pthread_library.md

@@ -80,8 +80,17 @@ Then build the program with this command:
 
 **Build with EMCC**
 
+> Note: This document is based on `emcc 2.0.26`, other version may not work with these commands
+
 EMCC's `-pthread` option is not compatible with standalone mode, we need to pass `-mbulk-memory -matomics` to the compiler and `--shared-memory,--no-check-features` to linker manually
 
+EMCC provides some empty implementation for pthread related APIs, we need to remove them from emcc's libc.
+``` bash
+cd ${emsdk_dir}/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten
+emar d libc.a library_pthread_stub.o
+emranlib libc.a
+```
+
 ``` bash
 emcc -O3 -mbulk-memory -matomics -s MALLOC="none"   \
      -Wl,--export=__data_end,--export=__heap_base   \
@@ -166,6 +175,8 @@ int pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex,
 
 int pthread_cond_signal(pthread_cond_t *cond);
 
+int pthread_cond_broadcast(pthread_cond_t *cond);
+
 int pthread_cond_destroy(pthread_cond_t *cond);
 
 /* Pthread key APIs */