For cpu_switchto(), inherit PSL_IMASK field of SR between lwp's.

Otherwise, IPL is lost during context switch, which allows improper
interrupts when, e.g., spin mutexes are hold.

With this fix, full ATF is successfully completed on DIAGNOSTIC
kernel (with one KASSERT in uvm_map.c, which triggers kern/51254,
converted to printf).

Thanks uwe@ for review and suggesting optimization.

Also thanks ad@ for useful comment, and ryo@ for giving me reference
manuals of SH-4!

(rin)