Add build flags to libwasm_workers

Author: juj

tools/system_libs.py

@@ -906,6 +906,8 @@ def can_use(self):
 class libwasm_workers(MTLibrary):
   name = 'libwasm_workers'
 
+  cflags = ['-Os', '-pthread']
+
   def get_files(self):
     return files_in_path(
         path='system/lib/wasm_worker',