dummy implementation of set_thread_area

eventually we may have a working "generic" implementation for archs
that don't need anything special. in any case, the goal of having
stubs like this is to allow early testing of new ports before all the
details needed for threads have been filled in. more functions like
this will follow.

src/thread/__set_thread_area.c

+#include <pthread_impl.h>
+
+int __set_thread_area(void *p)
+{
+	return -1;
+}