Added an OpenCL deinitialization test and plugin infrastructure.

[Kerilk]

Hello,

This PR does several things:

• Refactor platform specific components (and dependencies) of the loader in logical units;
• Creates a test program (cl_plugin_loader_test) that loads and unloads a shared library plugin in a loop (leverages above components);
• Creates a plugin, linked to the OpenCL ICD Loader that will exercise OpenCL calls when initialized through a predetermined entry point. When unloaded, this plugin will also automatically unload the OpenCL ICD Loader.
• Adds a new test that will invoke the cl_plugin_loader_test program using the plugin, exercising the OpenCL ICD Loader loading and unload capabilities.

For now the test just verifies that the program returns correctly, but additional manual testing can be done using loader tracing and memory validator like valgrind to ensure proper execution and absence of memory leaks.

[EwanC]

LGTM, my comments are all superficial

[Kerilk]

Thanks for the great review! I'll go an fix these ASAP.