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

*** 399,409 ****
                  xdr_mblk.o      \
                  xdr_mem.o       \
                  xdr.o           \
                  xdr_array.o     \
                  xdr_refer.o     \
-                 xhat.o          \
                  zone.o
  
  #
  #       Stubs for the stand-alone linker/loader
  #
--- 399,408 ----