cmd/oldlink: delete darwin/386 host object loading support

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

Updates #37610.

Change-Id: I11627c03403d4d4a3e0950fdedd0d850ae24b439
Reviewed-on: https://go-review.googlesource.com/c/go/+/227766


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