Support universal ("fat") Mach-Os

[woodruffw]

This isn't hard to do, I was just lazy about it:

1. If our input is a fat Mach-O, iterate over its architecture slices
2. Select the first x86-64 or i386 slice, preferring x86-64 over i386 if both are present

This could be made configurable (i.e., allowing the user to select the specific slice by its index), but it's probably not worth the effort.