Pull up following revision(s) (requested by martin in ticket #646):
sys/arch/alpha/alpha/vm_machdep.c: revision 1.114
Nowadays we can create new kernel threads late after boot, so if we are
unable to find physically continous pages for their uarea, just allocate
them via uvm_km_alloc(), that is: make cpu_uarea_alloc() fail instead
of panic. cpu_uarea_free() already deals with this.
Should fix PR port-alpha/53077.
Discussed with maxv and mrg.

(bouyer)