Support DW_EH_PE_sabsptr and 32-bit elf binaries

[EricRahm]

These changes should improve the support for 32-bit elf binaries. They add part of the handling for DW_EH_PE_sabsptr from #185. This got things further, but there were still issues. By wiring up the is_64bit flag rather than hardcoding it to true in a few places we get functioning output from bloaty with the example binary from #141:

% build/141/bloaty /tmp/issue141/bin -d compileunits
    FILE SIZE        VM SIZE
 --------------  --------------
   1.9%   319Ki  83.3%  18.1Mi    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/c/direct/image-direct.cpp
  45.6%  7.32Mi   8.1%  1.75Mi    [308 Others]
  15.4%  2.48Mi   0.0%       0    [section .symtab]
   6.7%  1.08Mi   0.5%   101Ki    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/c/gamepix.cpp
   3.7%   609Ki   0.7%   156Ki    /home/caiiiycuk/emscripten/monsters/common/classes/Field.cpp
   3.4%   557Ki   0.3%  69.3Ki    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/c/async.cpp
   0.2%  24.8Ki   2.4%   523Ki    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/zframework/zframework-1.5-pm-2012/zframework/gfx/nanojpeg.cpp
   3.0%   485Ki   2.2%   485Ki    [section .rodata]
   2.7%   442Ki   0.1%  28.2Ki    /home/caiiiycuk/emscripten/monsters/gpx/soundmgrimpl.cpp
   2.2%   360Ki   0.3%  75.0Ki    /home/caiiiycuk/emscripten/monsters/gpx/res.cpp
   1.9%   317Ki   0.1%  33.0Ki    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/c/fs.cpp
   1.9%   313Ki   0.2%  40.1Ki    /home/caiiiycuk/emscripten/monsters/gpx/iap.cpp
   1.7%   274Ki   0.2%  39.8Ki    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/zframework/zframework-1.5-pm-2012/zframework/visual/ScreenSizeMgr.cpp
   1.7%   273Ki   0.1%  31.4Ki    /home/caiiiycuk/emscripten/monsters/gpx/fontgenerator.cpp
   1.6%   265Ki   0.5%   102Ki    /home/caiiiycuk/emscripten/monsters/common/classes/GameController.cpp
   1.3%   206Ki   0.3%  61.1Ki    /home/caiiiycuk/emscripten/monsters/common/classes/GameScene.cpp
   1.2%   200Ki   0.1%  26.3Ki    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/c/storage.cpp
   1.1%   181Ki   0.1%  13.0Ki    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/c/profiler.cpp
   1.0%   159Ki   0.2%  36.8Ki    /home/caiiiycuk/emscripten/monsters/common/classes/MVSLPreferences.cpp
   1.0%   156Ki   0.3%  73.6Ki    /home/caiiiycuk/emscripten/monsters/common/classes/LevelSelectController.cpp
   0.9%   145Ki   0.1%  29.8Ki    /home/caiiiycuk/emscripten/sdk/gamepix/virtualenv/zframework/zframework-1.5-pm-2012/zframework/helpers/Banners/BannerSystemManager.cpp
 100.0%  16.0Mi 100.0%  21.7Mi    TOTAL

This also adds a test.

Fixes #141

[EricRahm]

@haberman This is a small change following up on issue #141. I rebased PR #185 and folded in your previous feedback as well as cleaning up some todo's around hardcoding is_64bit. I've included a lit test as well.

[haberman]

LGTM, thanks for the test!