Fix bugs in posix thread-id calculation when sizeof(pthread_t) != sizeof(long)

Fix bugs in posix thread-id calculation when sizeof(pthread_t) != sizeof(long)

When pthread_t was smaller, our calculated thread IDs would include
uninitialized RAM, and so our unit tests would fail because thread_ids
would never match one another.

When pthread_t was larger and alignment was big-endian, our calculated
thread IDs would only have the most significant bytes of the
pthread_t, when in practice all the entropy is in the low-order bytes.

Found with help from Dagobert Michelsen.