This commit adds scripts for running the WebAssembly binaries that the
Go compiler will produce.

The script go_js_wasm_exec uses Node.js to run the binaries. Adding it
to PATH will enable "go run" and "go test" to work for js/wasm
without having to manually provide the -exec flag.
See https://golang.org/cmd/go/#hdr-Compile_and_run_Go_program
for more information.

The web page wasm_exec.html is an example on how to run the same
binaries in a web browser.

Both scripts use wasm_exec.js as a shared library.

Updates #18892

Change-Id: Ia4d9bea025957750baa0d0651243dc88f156f85d
Reviewed-on: https://go-review.googlesource.com/103255Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>