lockdep/selftests: Adapt ww-tests for PREEMPT_RT

The ww-mutex selftest operates directly on ww_mutex::base and assumes
its type is struct mutex. This isn't true on PREEMPT_RT which turns the
mutex into a rtmutex.

Add a ww_mutex_base_ abstraction which maps to the relevant mutex_ or
rt_mutex_ function.
Change the CONFIG_DEBUG_MUTEXES ifdef to DEBUG_WW_MUTEXES. The latter is
true for the MUTEX and RTMUTEX implementation of WW-MUTEX. The
assignment is required in order to pass the tests.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>