as with the Block refcount changes, _xinc() and _xdec() arent
used anymore, so remove them.
architecure can still define ainc()/adec() when it needs them.
this simplifies the arm ports and keeps all the stuff in one place
instead of spreading it thru notify(), trap() and syscall() functions
and prevents useless fp state copying for kernel procs.
also make sure to save fp in notify while still splhi().
