cmd/compile: change size diagnostic to use benchmark format

Makes it easier to run

    go build -a -gcflags=-d=export std |& grep ^BenchmarkExportSize

and get useful output for feeding into benchstat.

Change-Id: I2b52e8f5ff33b7ccb6c25b18e464513344bd9ad9
Reviewed-on: https://go-review.googlesource.com/c/go/+/198698


Reviewed-by: Robert Griesemer <gri@golang.org>