locking: Separate rwlock api from spinlock api

Move the rwlock smp api defines and functions into a separate header
file. Makes the -rt selection simpler and less intrusive.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Ingo Molnar <mingo@elte.hu>