kernel: POSIX thread IPC support

Partial implementation of the IEEE 1003.1 pthread API, including
mutexes and condition variables in their default behaviors, and
pthread barrier objects.  The rwlock and spinlocks abstractions are
not supported in this commit (both only make sense in the presence of
multiple SMP processors).

Note that this is the IPC mechanisms only.  The thread creation API
itself is unsupported: Zephyr threads work differently from pthreads
and don't port cleanly in all cases.  Likewise the "_INITIALIZER"
macros from pthreads don't work cleanly here, and _DECLARE macros have
been provided to statically initialize pthread primitives in a manner
more native to Zephyr

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>