runtime: fix non-concurrent sweep
State of the world:
CL
46430043 introduced a new concurrent sweep but is broken.
CL
62360043 made the new sweep non-concurrent
to try to fix the world while we understand what's wrong with
the concurrent version.
This CL fixes the non-concurrent form to run finalizers.
This CL is just a band-aid to get the build green again.
Dmitriy is working on understanding and then fixing what's
wrong with the concurrent sweep.
TBR=dvyukov
CC=golang-codereviews
https://golang.org/cl/
62370043