Add extra suffix for HTML5 thread builds.

platform/javascript/detect.py

@@ -139,6 +139,7 @@ def configure(env):
         env.Append(LINKFLAGS=["-s", "USE_PTHREADS=1"])
         env.Append(LINKFLAGS=["-s", "PTHREAD_POOL_SIZE=4"])
         env.Append(LINKFLAGS=["-s", "WASM_MEM_MAX=2048MB"])
+        env.extra_suffix = ".threads" + env.extra_suffix
     else:
         env.Append(CPPDEFINES=["NO_THREADS"])