kernel - Remove cache ping-pong on common scheduler operations

kernel - Remove cache ping-pong on common scheduler operations

* Reflect dfly_curprocmask and dfly_rdyprocmask bits in the
  scheduler's pcpu structures.  This allows us to reduce global
  atomic ops that are virtually guaranteed to cause cache ping
  ponging.

* sched_yield and token-based yield operations no longer clear
  the bit in the curprocmask, since they are just yielding
  (XXX needs work, a later blocking op then might not pull a
  new process from another cpu).