go/build, go/types, cmd/dist: add js/wasm architecture

go/build, go/types, cmd/dist: add js/wasm architecture

This is the first commit of a series that will add WebAssembly
as an architecture target. The design document can be found at
https://docs.google.com/document/d/131vjr4DH6JFnb-blm_uRdaC0_Nv3OUwjEY5qVCxCup4.

The GOARCH name "wasm" is the official abbreviation of WebAssembly.
The GOOS name "js" got chosen because initially the host environment
that executes WebAssembly bytecode will be web browsers and Node.js,
which both use JavaScript to embed WebAssembly. Other GOOS values
may be possible later, see:
https://github.com/WebAssembly/design/blob/master/NonWeb.md

Updates #18892

Change-Id: Ia25b4fa26bba8029c25923f48ad009fd3681933a
Reviewed-on: https://go-review.googlesource.com/102835
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>