I noticed that some tests takes very long time on threaded SBCL on
FreeBSD 7. That was because the cost of busy loop is very large on the
system, especially with small HZ.
In such situations I usually insert sched_yield() on C, but we don't
have Lisp level interface.
So I wrote a function sb-thread:yield-current-thread to call sched_yield().
I think it's reasonable to have this interface.
Could someone review the attached patch?