Don't search for separate debug files for mach-o object files (#81041)

mach-o object files never have separate debug info, and in a big app
there can be quite a large number of object files, so even a few stats
per object file can slow launches considerably.
This patch avoids this search for Mach-o symbol files of object type.

I don't have a way to test this, the only effect is that you didn't do a
bunch of stats that weren't going to do any good anyway.