nptl: Add tests for internal pthread_rwlock_t offsets

nptl: Add tests for internal pthread_rwlock_t offsets

This patch new build tests to check for internal fields offsets for
internal pthread_rwlock_t definition.  Althoug the '__data.__flags'
field layout should be preserved due static initializators, the patch
also adds tests for the futexes that may be used in a shared memory
(although using different libc version in such scenario is not really
supported).

Checked with a build against all affected ABIs.

Change-Id: Iccc103d557de13d17e4a3f59a0cad2f4a640c148