cmd/link/internal/loadmacho: reduce scope of local declarations

cmd/link/internal/loadmacho: reduce scope of local declarations

Move some local declarations closer to their use, reducing their
respective lifetimes, also improve few error messages.
Follow up of CL 67370.

Updates #22095

Change-Id: I6131159ae8de571015ef5459b33d5c186e543a37
Reviewed-on: https://go-review.googlesource.com/69110
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>