We're working on building our Firefox Mac builds as a Linux cross-compile
(https://bugzilla.mozilla.org/show_bug.cgi?id=921040) and we need symbol
dumping to work. This change ports the Mac dump_syms tool to build and work
on Linux. I've tested it and it produces identical output to running the
tool on Mac.

The bulk of the work here was converting src/common/mac/dump_syms.mm and
src/tools/mac/dump_syms/dump_syms_tool.mm from ObjC++ to C++ and removing
their use of Foundation classes in favor of standard C/C++.

This won't compile out-of-the-box on Linux, it requires some Mac system
headers that are not included in this patch. I have those tentatively in
a separate patch to land in Gecko
(http://hg.mozilla.org/users/tmielczarek_mozilla.com/mc/rev/5fb8da23c83c),
but I wasn't sure if you'd be interested in having them in the Breakpad tree.
We could almost certainly pare down the set of headers included there, I
didn't spend too much time trying to ...