expvar: make BenchmarkAdd{Same,Different} comparable to 1.8

expvar: make BenchmarkAdd{Same,Different} comparable to 1.8

bradfitz noted in change 36717 that the new behavior was no longer
comparable with the old.  This change restores comparable behavior
for -cpu=1.

BenchmarkMapAddSame                 909           909           +0.00%
BenchmarkMapAddSame-6               1309          262           -79.98%
BenchmarkMapAddDifferent            2856          3030          +6.09%
BenchmarkMapAddDifferent-6          3803          581           -84.72%

updates #18177

Change-Id: Ifaff5a1f48be92002d86c296220313b7efdc81d6
Reviewed-on: https://go-review.googlesource.com/36723
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>