libgo: add riscv and js/wasm as known targets

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.

    Reviewed-on: https://go-review.googlesource.com/117996

From-SVN: r261451