libc - Add quick version for the context management functions.

libc - Add quick version for the context management functions.

* Add makecontext_quick(), setcontext_quick(), and swapcontext_quick().
  These functions work similarly to the non-quick versions but are designed
  for fast synchronous switching.  These functions do not mess with the
  signal mask or stack at all and do not save or restore scratch registers.

* These functions make no system calls.  Switching time can be as low as
  ~5 nanoseconds.

* These functions also provide optimizations for coroutine fall-through
  linkages.

* Note that the coroutine / start-function callback arguments are somewhat
  different.  Start functions are called back as cofunc(ucp, arg).  Var-args
  are not supported and the stack is minimally aligned and initialized.

* Remove the old internal set_mcontext() and get_mcontext() routines.