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

@@ -79,11 +79,10 @@
 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