runtime: ensure consistency of /gc/scan/*

runtime: ensure consistency of /gc/scan/*

Currently /gc/scan/total:bytes is computed as a separate sum. Compute it
using the same inputs so it's always consistent with the sum of
everything else in /gc/scan/*.

For #56857.

Change-Id: I43d9148a23b1d2eb948ae990193dca1da85df8a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/497880
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>