[dev.link] cmd/link: delete darwin/386 host object loading support

In Go 1.15, the darwin/386 port is gone.

Updates #37610.

Change-Id: I308561848d90d9a649b3603ab1f7fc1117305ec1
Reviewed-on: https://go-review.googlesource.com/c/go/+/227763


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