runtime: switch p.gcFractionalMarkTime to atomic.Int64

runtime: switch p.gcFractionalMarkTime to atomic.Int64

atomic.Int64 automatically maintains proper alignment, avoiding the need
to manually adjust alignment back and forth as fields above change.

Change-Id: I6a6a636c4c3c366353f6dc8ecac473c075dd5cd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/716700
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>