cmd/compile: add a flag to print the source line for generated rules

When working on rulegen, I often find myself
searching the rules files to find the source of
generated code. Add a flag to make that easier.

The flag needs to be off by default,
so that adding a single rule doesn't cause a massive diff.

Change-Id: I5a6f09129dc6fceef7c9cd1ad7eee24f3880ba91
Reviewed-on: https://go-review.googlesource.com/c/go/+/213700


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>