libthr: Use TLS_TCB_* in _tcb_[cd]tor.

This matches libc and rtld in using the alignment (TLS_TCB_ALIGN) from
machine/tls.h instead of hardcoding 16.

Reviewed by: kib
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D34023