Fixes #18700 (backport)

Currently we check that all roots are marked as soon as gcMarkDone
decides to transition from mark 1 to mark 2. However, issue #16083
indicates that there may be a race where we try to complete mark 1
while a worker is still scanning a stack, causing the root mark check
to fail.

We don't yet understand this race, but as a simple mitigation, move
the root check to after gcMarkDone performs a ragged barrier, which
will force any remaining workers to finish their current job.

Updates #16083. This may "fix" it, but it would be better to
understand and fix the underlying race.

Change-Id: I1af9ce67bd87ade7bc2a067295d79c28cd11abd2
Reviewed-on: https://go-review.googlesource.com/35678
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>