kernel: sched: Change cpu pin only for not executing threads

Do not allow changing the CPU which a thread is pinned when it is
already being executed. This allows further optimizations in some
platforms with incoherent memory since we can safely assume that the
thread will run in the same CPU and avoid invalidate / flush the
cache during context switches.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>