cmd/compile: fix mknode script

cmd/compile: fix mknode script

It's not currently working. Somehow a field of type []constant.Value
causes it to barf. (That field was added with jump table statements.)

Also added some instructions about how to run it correctly (which took
me a suprisingly long time to figure out).

Larger improvements coming, but this gets us to a working state
and is safe for 1.19.

Change-Id: I3027356fde1294942e87d075ca28bb40d2c0d6c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/418234
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Keith Randall <khr@golang.org>