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
remove zulu (XVR-4000)
XVR-4000 was a very expensive, very rare graphics card.

*** 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 ----
*** 133,143 ****
  PMC_OBJS        += pmc.o
  TRAPSTAT_OBJS   += trapstat.o
  I2BSC_OBJS      += i2bsc.o
  GPTWOCFG_OBJS   += gptwocfg.o
  GPTWO_CPU_OBJS  += gptwo_cpu.o
- ZULUVM_OBJS     += zuluvm.o zulu_asm.o zulu_hat.o zulu_hat_asm.o
  
  JBUSPPM_OBJS    += jbusppm.o 
  RMC_COMM_OBJS   += rmc_comm.o rmc_comm_crctab.o rmc_comm_dp.o rmc_comm_drvintf.o
  RMCADM_OBJS     += rmcadm.o
  MEM_CACHE_OBJS  += mem_cache.o panther_asm.o
--- 132,141 ----