driver: report benchmem in benchmark name

The benchmem value is important to understanding the results of the
garbage benchmark, so it's important to include it in the benchmark
report.

Change-Id: I6bf2b0c8450b9c4e34d39a7f12822870f5fad131
Reviewed-on: https://go-review.googlesource.com/33583
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>