cmd/go: stub out gotoolchain.go for wasip1 os build tag
There's a stub for gotoolchain.go for the js build tag because js/wasm
doesn't define syscall.Exec. But there are builders that are wasm but
not js, which also don't have syscall.Exec. The wasip1 GOOS is one
example. Stub out gotoolchain.go for wasip1 also.
Change-Id: I224bb385474ad9c5d3c28a83a000f450dfb43c0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/485735
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>