cmd/internal/obj: make Prog.From3 a pointer

cmd/internal/obj: make Prog.From3 a pointer

It is almost never set and Addr is large, so having the full struct
in the Prog wastes memory most of the time.

Before (on a 64-bit system):

$ sizeof -p cmd/internal/obj Addr Prog
Addr 80
Prog 376
$

After:

$ sizeof -p cmd/internal/obj Addr Prog
Addr 80
Prog 304
$

Change-Id: I491f201241f87543964a7d0f48b85830759be9d0
Reviewed-on: https://go-review.googlesource.com/10457
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>