cmd/bent: makeover for benchdwarf & benchsize; add assume=exact for units

Also made these slightly more robust against missing data,
and corrected a bad field reference for args-quality benchmarking.

Change-Id: I5a561d89fac2285e5e23c9e7e730532c3f26a5c6
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/386698
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
2 files changed