cmd/go/internal/mmap: fix build for js/wasm

cmd/go/internal/mmap: fix build for js/wasm

Even though there isn't much point in building cmd/go for js/wasm given
it relies on executing sub-processes, this change is trivial enough and
makes cmd/go build for js/wasm again.

Fixes #54219

Change-Id: I91e3adec0f31a4e261af89b4cc829711828ac2ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/433480
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>