sched: add do_sched_yield() helper; remove in-kernel call to sched_yield()

sched: add do_sched_yield() helper; remove in-kernel call to sched_yield()

Using the sched-internal do_sched_yield() helper allows us to get rid of
the sched-internal call to the sys_sched_yield() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527[email protected]

Cc: Ingo Molnar <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Dominik Brodowski <[email protected]>