Create loader infrastructure for constructing the payloads of external
symbols from scratch, as opposed to passing in a sym.Symbol object
containing the payload.

The general idea is that clients can use the loader to create new
external Sym's using loader.AddExtSym, and then can add
relocations/data to the new sym with symbol builder interfaces (to be
provided in an subsequent patch), as opposed to having to use
sym.Symbol.

This change preserves compatibility with the old way of doing things
(passing in sym.Symbol) via a new loader.InstallSym method. If a
client invokes this method for a specific Sym, then the loader keeps
track of this fact and uses the sym.Symbol as the backing store instead.

Also included is a small unit test for the new interfaces -- not clear
whether this really needs to be kept around long term... it was mainly
useful during initial bringup.

Change-Id: If8ab15df7b64636e56b317155dfe6d7cdfe23b71
Reviewed-on: https://go-review.googlesource.com/c/go/+/207606


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