remove xhat
The xhat infrastructure was added to support hardware such as the zulu
graphics card - hardware which had on-board MMUs.  The VM used the xhat code
to keep the CPU's and Zulu's page tables in-sync.  Since the only xhat user
was zulu (which is gone), we can safely remove it simplifying the whole VM
subsystem.
Assorted notes:
- AS_BUSY flag was used solely by xhat

*** 62,72 ****
  CORE_OBJS +=    memscrub.o
  CORE_OBJS +=    memscrub_asm.o
  CORE_OBJS +=    ppage.o
  CORE_OBJS +=    sfmmu_kdi.o
  CORE_OBJS +=    swtch.o
- CORE_OBJS +=    xhat_sfmmu.o
  
  #
  #       Some objects must be linked at the front of the image (or
  #       near other objects at the front of the image).
  #
--- 62,71 ----