mach relocation parsing doesn't handle big endian

The relocation fields are bitfields, which makes the ordering all weird. Currently we only use the little endian ordering, but that won't work for big endian files.

LLVM has code to handle this difference, see [here](https://github.com/llvm-mirror/llvm/blob/2c4ca6832fa6b306ee6a7010bfb80a3f2596f824/lib/Object/MachOObjectFile.cpp#L141-L170).

Also related, we don't currently parse scattered relocations.

Start by reviewing the relocation parsing implementation and the LLVM Mach-O reference linked in the issue, focusing on how bitfield ordering differs between little- and big-endian files. Verify behavior with big-endian Mach-O input, then assess the related scattered-relocation parsing gap; done means both relocation forms are parsed correctly.

Written by the indexing model from the issue text.