Implement pthread_condattr_{get,set}clock.

We only support CLOCK_REALTIME and CLOCK_MONOTONIC for now,
so we us a single bit from pthread_cond_t->value to denote
the clock type. Note that this reduces the width of the counter
to 30 bits, but this should be large enough for all practical
purposes.

bug: 13232338

Change-Id: I857e7da64b3ecbb23eeac7c9f3fbd460f60231bd