add porting codes of rt-thread (#494)

core/iwasm/libraries/lib-pthread/SConscript

@@ -0,0 +1,20 @@
+#
+# Copyright (c) 2021, RT-Thread Development Team
+#
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+
+from building import *
+
+cwd     = GetCurrentDir()
+
+src = Split('''
+libc_pthread_wrapper.c
+''')
+
+CPPPATH = [cwd]
+
+
+group = DefineGroup('iwasm_libc_pthread', src, depend = [''], CPPPATH = CPPPATH)
+
+Return('group')