libroot: Add support for C11 Threads missing parts (except gcc2).

The respective files can be found in the FreeBSD source tree at:
- lib/libstdthreads/call_once.c
- lib/libstdthreads/cnd.c
- lib/libstdthreads/mtx.c
- lib/libstdthreads/threads.h
- lib/libstdthreads/tss.c

Missing is support for PTHREAD_DESTRUCTOR_ITERATIONS.

Change-Id: I7a6c79954f36195eadd1351d308c21a001192232
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5675
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>