[dev.link] cmd/link: invoke oldlink if old object format is chosen

[dev.link] cmd/link: invoke oldlink if old object format is chosen

Now we can choose the old object file format by setting
-gcflags=all=-go115newobj=false -asmflags=all=-go115newobj=false -ldflags=all=-go115newobj=false

Tested that setting all three to default false and it still works.

Change-Id: I9514b62a676916cc383b8afa389489fe7b8fa2bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/224625
Reviewed-by: Than McIntosh <thanm@google.com>