cmd/link: use standard-syntax struct tags in large-tag test

These ridiculous tags are testing what happens with very long tags.
There is no need for them to be malformed as well.
Fix them to make vet happier.

Working toward making the tree vet-safe instead of having
so many exceptions in cmd/vet/all/whitelist.

For #31916.

Change-Id: I100aed5230fcde41676c79c7074c69c16ea4b96d
Reviewed-on: https://go-review.googlesource.com/c/go/+/176111


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>