Fixed 'undefined reference to __get_thread' errors.

Compiling libbthread using build.sh resulted in undefined reference to '__get_thread' errors.
This allows it to build correctly and will hopefully work.

Note: pthread_self() is defined as

    pthread_t pthread_self(void)
    {
        return (pthread_t)__get_thread();
    }

in libc/bionic/pthread.c