sched_ule: mark scheduler interface functions as weak symbols

[minsoochoo0122_proton.me]

Summary

When a kernel module scheduler is loaded, its sched_* implementation
will override GENERIC's weak sched interface.

As part of this effort, two more changes are included in this review:

sched_ule: add dummy sched_pctcpu_delta
Rename __inline to inline and reorder some static inline functions in sched.h

Test Plan

GENERIC build and boot succeeded.

[minsoochoo0122_proton.me]

sys/kern/sched_ule.c

3019

ULE doesn't need this, and personally I don't think any modern scheduler would require this. Should we provide this as <sys/sched_4bsd.h>?