kernel/sched: protect thread sched_lock with compiler barriers

This has not bitten us yet, but it was a ticking timebomb.

This is similar to the issue that was found with irq_lock/irq_unlock
implementations on several architectures. Having a volatile variable is
not the way to force the sched_lock variable to be
incremented/decremented around the accesses to data it protects.
Instead, a compiler barrier must prevent the compiler from reordering
the memory accesses around setting of sched_lock. Needed in the inline
implementations _sched_lock()/_sched_unlock_no_reschedule(), which
resolve to simple decrement/increment of the per-thread sched_lock
variable.

Change-Id: I06f5b3524889f193efe69caa947118404b1be0b5
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>