This is a separate step from generating and posting BugSplat symbols, since
BugSplat needs the executable along with the symbols, and we don't need to
consume that space in a symbols tarball.

Move Mac BugSplat symbol generation logic to CMake land, the same general area
where Breakpad symbols are generated. Add stanzas to pack up the usual tarball
for Windows and Mac.

Remove the build.sh test that suppressed uploading the symbols tarball for
BugSplat builds.