nptl: Cleanup mutex internal offset tests

The offsets of pthread_mutex_t __data.__nusers, __data.__spins,
__data.elision, __data.list are not required to be constant over
the releases.  Only the __data.__kind is used for static
initializers.

This patch also adds an additional size check for __data.__kind.

Checked with a build against affected ABIs.

Change-Id: I7a4e48cc91b4c4ada57e9a5d1b151fb702bfaa9f