Make uselocale(3) claim its pthread key in an ELF constructor.

pthread_once is nice for decoupling, but it makes resource availability less
predictable, which is a bad thing.

This fixes a test failure if uselocale(3) is called before
pthread.pthread_key_create_lots runs.

Change-Id: Ie2634f986a50e7965582d4bd6e5aaf48cf0d55c8