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

@@ -399,11 +399,10 @@
                 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
 #