cmd/link: refactor setCgoAttr

setCgoAttr takes a lookup function, but there's only a single call and
setCgoAttr already has access to the lookup function passed at that
call. Simplify setCgoAttr by eliminating the lookup parameter and
calling the lookup function directly.

For #40724.

Change-Id: Ib27c0fa2b88c387e30423365f7757e3ba02cf7d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/309338


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>