This commit allows architectures to disable optimizations that need the
Avg* and Hmul* operations.

WebAssembly has no such operations, so using them as an optimization
but then having to emulate them with multiple instructions makes no
sense, especially since the WebAssembly compiler may do the same
optimizations internally.

Updates #18892

Change-Id: If57b59e3235482a9e0ec334a7312b3e3b5fc2b61
Reviewed-on: https://go-review.googlesource.com/103256
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>