libgo: add riscv and js/wasm as known targets

Incorporates cut down versions of https://golang.org/cl/102835 and
https://golang.org/cl/106256 from the master sources.

This will tell go/build to skip files with those tags.

Change-Id: Idd0fa240e5ecf21d8773ce30bf239b208b4d1ef2
Reviewed-on: https://go-review.googlesource.com/117996
Reviewed-by: Than McIntosh <thanm@google.com>
5 files changed