This reverts commit cc514d7791dae0808124249ff545d9dda39b24b7.
* It introduced a regression.
https://github.com/apache/incubator-nuttx/issues/3868
* It seems conceptually wrong to have per-process data in
the main thread's stack.
1. Move pthread-specific data files from sched/pthread/ to libs/libc/pthread.
2. Remove pthread-specific data functions from syscalls.
3. Implement tls_alloc() and tls_free() with system calls.
4. Reimplement pthread_key_create() and pthread_key_free() using tls_alloc() and tls_free().
5. Reimplement pthread_set_specific() and pthread_get_specicif() using tls_set_value() and tls_get_value()
