This change adds the GOWASM option "signext" to enable
the generation of experimental sign-extension operators.

The feature is in phase 4 of the WebAssembly proposal process:
https://github.com/WebAssembly/meetings/blob/master/process/phases.md

More information on the feature can be found at:
https://github.com/WebAssembly/sign-extension-ops/blob/master/proposals/sign-extension-ops/Overview.md

Change-Id: I6b30069390a8699fbecd9fb4d1d61e13c59b0333
Reviewed-on: https://go-review.googlesource.com/c/go/+/168882Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>