Fix Mac Breakpad host tools to build in Linux cross-compile

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 minimize them (we primarily just need
the Mach-O structs and a few associated bits).

I just realized that this patch is missing updating the XCode project files
(ugh). I'll fix that up in a bit.

R=mark@chromium.org
BUG=https://bugzilla.mozilla.org/show_bug.cgi?id=543111

Review URL: https://codereview.chromium.org/1340543002 .

src/common/stabs_reader.h

@@ -53,11 +53,10 @@
 #include <config.h>
 #endif
 
-#ifdef HAVE_A_OUT_H
-#include <a.out.h>
-#endif
 #ifdef HAVE_MACH_O_NLIST_H
 #include <mach-o/nlist.h>
+#elif defined(HAVE_A_OUT_H)
+#include <a.out.h>
 #endif
 
 #include <string>