cmd/link/internal/ld: consolidate macho platform setup

cmd/link/internal/ld: consolidate macho platform setup

Determine the macho platform once and use that the two places that
need it. This makes it easier to add a third platform check for a
follow-up change.

Updates #31447

Change-Id: I522a5fface647ab8e608f816c5832d531534df7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/174198
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>