encoding/gob: report allocs in benchmarks

encoding/gob: report allocs in benchmarks

I almost exclusively use these benchmarks with -benchtime already.

Change-Id: I6539cbba6abbdb6b275502e122f4e16856d8b9e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/481375
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Rob Pike <r@golang.org>