support cputime clocks for processes/threads other than self

apparently these features have been in Linux for a while now, so it
makes sense to support them. the bit twiddling seems utterly illogical
and wasteful, especially the negation, but that's how the kernel folks
chose to encode pids/tids into the clock id.

src/thread/pthread_getcpuclockid.c

@@ -2,5 +2,6 @@
 
 int pthread_getcpuclockid(pthread_t t, clockid_t *clockid)
 {
-	return ENOSYS;
+	*clockid = (-t->tid-1)*8U + 6;
+	return 0;
 }