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>

Author: mdempsky

src/cmd/compile/internal/gc/export.go

@@ -71,7 +71,7 @@ func dumpexport(bout *bio.Writer) {
 	exportf(bout, "\n$$\n")
 
 	if Debug_export != 0 {
-		fmt.Printf("export data size = %d bytes\n", size)
+		fmt.Printf("BenchmarkExportSize:%s 1 %d bytes\n", myimportpath, size)
 	}
 }