mixed-mode *nix support: 64-bit + 32-bit on *nix

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on January 02, 2014 01:08:26_

Xref issue #49 , issue #828 , issue #865 , etc. for Windows mixed-mode and for mixed-mode
client issues.

On both Mac and Linux, mixing libraries not officially supported. Also on
both, there's no WOW-like user-mode layer: 32-bit just talks directly to
the kernel. We have yet to see any app that mixes modes. For now we do
not plan to support mixed-mode except on Windows. We certainly don't plan
to support mixing 32-bit and 64-bit libraries: we are currently hardcoding
the ELF and Mach-O struct definitions for the current arch.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1345_