Currently these two forms of layout are done in a single pass. This
makes it difficult to compress DWARF sections because that must be
done after relocations are applied, which must happen after virtual
address layout, but we can't layout the file until we've compressed
the DWARF sections.

Fix this by separating the two layout steps. In the process, we can
also unify the copy-pasted code in Link.address to compute file
offsets. Currently, each instance of this is slightly different, but
there's no reason for it to be. For example, we don't perform
PEFILEALIGN alignment on Segrodata or Selreltodata even when HeadType
== Hwindows, but it turns out it doesn't matter whether you do or
don't because these segments simply don't exist on Windows. Hence, in
the unified code path, we do this alignment for all segments.
Likewise, there are two ways of computing Fileoff:
  seg.Vaddr - prev.Vaddr + prev.Fileoff
and
  prev.Fileoff + uint64(Rnd(int64(prev.Filelen), int64(*FlagRound)))
At the moment, these always have the same value, but the latter will
continue to work after we start compressing sections on disk.

Tested by comparing test binaries for all packages in std before and
after this change for GOOS={linux,windows,darwin,plan9}. All binaries
are identical.

For #11799.

Change-Id: If09f28771bb4d78dd392fd58b8d7c9d5f22b0b9f
Reviewed-on: https://go-review.googlesource.com/111682
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>