debugging tools for *nix platforms

This has been sitting in one of my topic branches for a while, they're a few debugging tools that it might be useful to have sitting around in `base`.

* `Sys.shlib_list()` iterates through the shared libraries linked into our current process, returning a list of their paths.  Useful when you're not sure what `dlopen()` is picking up.

* Also adds in some simple logic to get some info about the CPU we're running on beyond whether it's 32 or 64-bit.