- thread_pthread.c (get_stack): pthread_attr_getstack() doesn't return stack start addres, but stack base address. Thus, we need to add stack size for getting stack start address. And, we don't have to decrease guard size twice.
- thread_pthread.c (thread_start_func_1): don't use inaccurate stack start guess if native_thread_init_stack() can be used. [Bug #1813] [ruby-core:24540]