Add helper to locate SLA files

[tetsuo-cpp]

As per @pgoodman's comment at #9 (comment), we should look into a way of exposing a helper that finds SLA files in the SLEIGH installation that it belongs to. Ideally it should just look like this:

/*
* Returns a string containing the path to the .sla file for a given architecture.
* @param arch: The architecture for which we want to find a .sla file
* @return: An absolute path to the .sla file
* @throw std::invalid_argument: An unknown arch is provided.
*/
std::string FindSlaFile(const std::string& arch);

[ekilmer]

I'm curious how you are planning to do this? Ideally this function would work when the linked library (or executable being run) is in the build directory and after it is installed somewhere. The build directory path can be a hard-coded (hard-calculated) absolute path using CMake, but the install path should be relative to the executable or library so as to have a relocatable package.

I think LLVM has some machinery to do this in clang to find libcxx headers 🤔

[pgoodman]

The way in Remill is to embed the CMAKE_INSTALL_PREFIX-related location, the build folder location, and then some random search paths. We could use configure_file to create a bunch of paths in a header file, then search down a list. There should also be the ability to pass in a list of paths to prefer as hints.