Have cpu_throw() loop on blocked_lock as well. This bug has existed
a long time and has gone unnoticed just as long, because I kept
using sched_4bsd (due to sched_ule not working with preemption),
but GENERIC had sched_ule by default -- including SMP.

While here, remove unused inclusion of <machine/clock.h>, remove
totally bogus inclusion of <i386/include/specialreg.h>.