cmd/internal/obj: improve obj.ProgInfo struct layout

cmd/internal/obj: improve obj.ProgInfo struct layout

obj.ProgInfo is a field inside obj.Prog, which is currently 320 bytes
on 64bit platforms. By moving the Flags field below the other fields
the size of obj.Prog drops into the 288 byte size class, a saving of
32 bytes per value allocated on the heap.

Change-Id: If8bb12f45328996d7df1d0bac9d1c019d2af73bd
Reviewed-on: https://go-review.googlesource.com/15522
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>