[dev.link] cmd/link: create loader-specific version of GCProg

[dev.link] cmd/link: create loader-specific version of GCProg

Create a new version of the GCProg type + methods that use loader APIs
instead of sym.Symbol.

This code isn't actually used just yet, but will be needed once the
wavefront reaches dodata() and we need to convert that phase.

Change-Id: I087521832015818204fe5c2ac99c7bd3f61b2bf0
Reviewed-on: https://go-review.googlesource.com/c/go/+/229037
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>