Make preparations for increasing the size of the kernel virtual address space

on the amd64 architecture.  The amd64 architecture requires kernel code and
global variables to reside in the highest 2GB of the 64-bit virtual address
space.  Thus, the memory allocated during bootstrap, before the call to
kmem_init(), starts at KERNBASE, which is not necessarily the same as
VM_MIN_KERNEL_ADDRESS on amd64.

sys/vm/vm_kern.c

@@ -497,8 +497,12 @@ kmem_init(start, end)
 	/* N.B.: cannot use kgdb to debug, starting with this assignment ... */
 	kernel_map = m;
 	(void) vm_map_insert(m, NULL, (vm_ooffset_t) 0,
-	    VM_MIN_KERNEL_ADDRESS, start, VM_PROT_ALL, VM_PROT_ALL,
-	    MAP_NOFAULT);
+#ifdef __amd64__
+	    KERNBASE,
+#else		     
+	    VM_MIN_KERNEL_ADDRESS,
+#endif
+	    start, VM_PROT_ALL, VM_PROT_ALL, MAP_NOFAULT);
 	/* ... and ending with the completion of the above `insert' */
 	vm_map_unlock(m);
 }