so_pru_send_async: Use M_NOWAIT to allocate address

If the allocation failed, we just fall back to the synchronized pru_send.
Using M_WAITOK defeats the purpose of asynchronized pru_send, which tries
to prevent the current thread from being scheduled.
kernel: Remove Cyrix CPUs specific handling

* Remove kernel options, constants, detection code and special
  initialization routines for Cyrix CPUs

* Part of this code prevents the i386 vkernel from beeing compiled
  from a source tree with PAT support

* Recent versions of DragonFly cannot possibly run on machines using
  these processors anyway