This commit adds the wasm architecture to the link command.

Design doc: https://docs.google.com/document/d/131vjr4DH6JFnb-blm_uRdaC0_Nv3OUwjEY5qVCxCup4

Updates #18892

Change-Id: I5aef29954984537f2979679b5d393209e462f564
Reviewed-on: https://go-review.googlesource.com/103795
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>