cmd/link: rewrite some code without using reflection

In Mach-O DWARF combining, some code was written using reflection,
so it could support both 32-bit and 64-bit Mach-O files without
duplicating code. We no longer support 32-bit darwin platforms
now. 32-bit support can go. Rewrite it with direct field access,
for 64-bit only.

Change-Id: If1338c3cd37cecf603f4df0c6eb0c890eaebfe5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/253557


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>