[dev.link] cmd/link/internal/objfile: new 'Sym' type for global symbol index

[dev.link] cmd/link/internal/objfile: new 'Sym' type for global symbol index

First change of several to update the loader API to reflect the final
consensus version of the loader API as described in Cherry's doc.

This piece:

 - define new loader.Sym type to encapsulate a global symbol
   index (as opposed to just using 'int')

Change-Id: I6f6483e269f80abfc7d373b2856b2c0d61b9ac24
Reviewed-on: https://go-review.googlesource.com/c/go/+/200417
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>