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

*** 79,89 ****
  CORE_OBJS +=    promif_version.o
  CORE_OBJS +=    sfmmu_kdi.o
  CORE_OBJS +=    suspend.o
  CORE_OBJS +=    swtch.o
  CORE_OBJS +=    wdt.o
- CORE_OBJS +=    xhat_sfmmu.o
  
  CORE_OBJS +=    mdesc_diff.o
  CORE_OBJS +=    mdesc_findname.o
  CORE_OBJS +=    mdesc_findnodeprop.o
  CORE_OBJS +=    mdesc_fini.o
--- 79,88 ----