The interface between the wasm binary and wasm_exec.js is experimental
and likely to change in the future. Still, there are some early adopters
who experiment with non-web wasm runtimes. They can't use wasm_exec.js
and have to provide their own equivalent. Adding the Go version as a
custom wasm sections allows for them to support a stable Go version and
the latest devel at the same time.

Change-Id: I6d377bb0a0c33cb80e86dd15a34ddc9a70680227
Reviewed-on: https://go-review.googlesource.com/127597
Run-TryBot: Richard Musiol <neelance@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>