In preparation for being able to run a go program that has code
in several objects, this changes from having several linker
symbols used by the runtime into having one linker symbol that
points at a structure containing the needed data.  Multiple
object support will construct a linked list of such structures.

A follow up will initialize the slices in the themoduledata
structure directly from the linker but I was aiming for a minimal
diff for now.

Change-Id: I613cce35309801cf265a1d5ae5aaca8d689c5cbf
Reviewed-on: https://go-review.googlesource.com/7441Reviewed-by: Ian Lance Taylor <iant@golang.org>