cmd/link/internal/ld: reduce the scope of some local declarations

cmd/link/internal/ld: reduce the scope of some local declarations

Move some local declarations closer to their use, reducing their
respective lifetimes. Spotted while reviewing CL 67318.

Change-Id: I68db67fe8530344d95e50efb6587bc724e1171f5
Reviewed-on: https://go-review.googlesource.com/67370
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>