The flakiness appears to be just in tests, not in the actual code.
Specifically, the many tests call runtime.GC once and expect that
the finalizers will be running in the background when GC returns.
Now that the sweep phase is concurrent with execution, however,
the finalizers will not be run until sweep finishes, which might
be quite a bit later. To force sweep to finish, implement runtime.GC
by calling the actual collection twice. The second will complete the
sweep from the first.

This was reliably broken after a few runs before the CL and now
passes tens of runs:

while GOMAXPROCS=2 ./runtime.test -test.run=Finalizer -test.short \
        -test.timeout=300s -test.cpu=$(perl -e 'print ("1,2,4," x 100) . "1"')
do true; done

Fixes #7328.

LGTM=dvyukov
R=dvyukov, dave
CC=golang-codereviews
https://golang.org/cl/71080043