Basic Mach-O support in file(1).

The Nexus Player build was subtly broken in that it assumed that the host was
using ELF. No-one noticed until a Mac user tried to flash their build, which
contained a Mach-O x86 binary instead of an ELF x86 binary. Hilarity ensued.

(On the same day, file(1) was able to explain a mixup with an ELF hexagon
binary. Next time we see a Mach-O binary on an Android device, we'll be ready!)

Bug: http://b/66741960