Add pthread_gettid_np and re-expose __get_thread for LP32.

Add pthread_gettid_np and re-expose __get_thread for LP32.

A lot of third-party code calls the private __get_thread symbol,
often as part of a backport of bionic's pthread_rwlock implementation.
Hopefully this will go away for LP64 (since you're guaranteed the
real implementation there), but there are still APIs that take a tid
and no way to convert between a pthread_t and a tid. pthread_gettid_np
is a public API for that. To aid the transition, make __get_thread
available again for LP32.

(cherry-pick of 27efc48814b8153c55cbcd0af5d9add824816e69.)

Bug: 14079438
Change-Id: I43fabc7f1918250d31d4665ffa4ca352d0dbeac1