sched/core: Move preempt_model_*() helpers from sched.h to preempt.h

Move the declarations and inlined implementations of the preempt_model_*()
helpers to preempt.h so that they can be referenced in spinlock.h without
creating a potential circular dependency between spinlock.h and sched.h.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ankur Arora <ankur.a.arora@oracle.com>
Link: https://lkml.kernel.org/r/20240528003521.979836-2-ankur.a.arora@oracle.com