Forked from CL 111895.

The trickiest part of this is that the binary layout code (blk,
elfshbits, and various other things) assumes a constant offset between
symbols' and sections' file locations and their virtual addresses.
Compression, of course, breaks this constant offset. But we need to
assign virtual addresses to everything before compression in order to
resolve relocations before compression. As a result, compression needs
to re-compute the "address" of the DWARF sections and symbols based on
their compressed size. Luckily, these are at the end of the file, so
this doesn't perturb any other sections or symbols. (And there is, of
course, a surprising amount of code that assumes the DWARF segment
comes last, so what's one more place?)

Relevant benchmarks:
name        old time/op     new time/op     delta
StdCmd          10.3s ± 2%      10.8s ± 1%   +5.43%  (p=0.000 n=30+30)

name        old text-bytes  new text-bytes  delta
HelloSize       746kB ± 0%      746kB ± 0%     ~     (all equal)
CmdGoSize      8.41MB ± 0%     8.41MB ± 0%     ~     (all equal)
[Geo mean]     2.50MB          2.50MB        +0.00%

name        old data-bytes  new data-bytes  delta
HelloSize      10.6kB ± 0%     10.6kB ± 0%     ~     (all equal)
CmdGoSize       252kB ± 0%      252kB ± 0%     ~     (all equal)
[Geo mean]     51.5kB          51.5kB        +0.00%

name        old bss-bytes   new bss-bytes   delta
HelloSize       125kB ± 0%      125kB ± 0%     ~     (all equal)
CmdGoSize       145kB ± 0%      145kB ± 0%     ~     (all equal)
[Geo mean]      135kB           135kB        +0.00%

name        old exe-bytes   new exe-bytes   delta
HelloSize      1.60MB ± 0%     1.05MB ± 0%  -34.39%  (p=0.000 n=30+30)
CmdGoSize      16.5MB ± 0%     11.3MB ± 0%  -31.76%  (p=0.000 n=30+30)
[Geo mean]     5.14MB          3.44MB       -33.08%

Fixes #11799.
Updates #6853.

Change-Id: I64197afe4c01a237523a943088051ee056331c6f
Reviewed-on: https://go-review.googlesource.com/118276
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>