cmd/compile: rework mkbuiltin.go to generate code

cmd/compile: rework mkbuiltin.go to generate code

Generating binary export data requires a working Go compiler. Even
trickier to change the export data format itself requires a careful
bootstrapping procedure.

Instead, simply generate normal Go code that lets us directly
construct the builtin runtime declarations.

Passes toolstash -cmp.

Fixes #17508.

Change-Id: I4f6078a3c7507ba40072580695d57c87a5604baf
Reviewed-on: https://go-review.googlesource.com/31493
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>