cmd/go: add GOWASM environment variable

This change adds the environment variable GOWASM, which is a comma
separated list of experimental WebAssembly features that the compiled
WebAssembly binary is allowed to use. The default is to use no
experimental features. Initially there are no features avaiable.

More information about feature proposals can be found at
https://github.com/WebAssembly/proposals

Change-Id: I4c8dc534c99ecff8bb075dded0186ca8f8decaef
Reviewed-on: https://go-review.googlesource.com/c/go/+/168881


Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>