all: remove darwin/386 build-tags and files

This removes all files that are only used on darwin/386 and cleans up
build tags in files that are still used on other platforms.

Updates #37610.

Change-Id: If246642476c12d15f59a474e2b91a29c0c02fe75
Reviewed-on: https://go-review.googlesource.com/c/go/+/227581


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>