Introduce a platform abstraction layer

[andreabedini]

Different architectures are supported by having #if defined scattered around the codebase. Introducing a minimal platform abstraction API will simplify the code and lower its maintainance cost. E.g. turning

#if defined(mingw32_HOST_OS)
someWindowsFunction();
#else
someUnixFunction();
#endif

into

platformSpecificFunction();

Where platformSpecificFunction is implemented in a separate file that can included only when relevant. Implementing the same API will also make sure all supported architectures provide the same features.

This is rough sketch of platform-dependent APIs:

Function Category	Needed API Functions
Thread Management	platform_create_thread(), platform_join_thread()
Process Management	platform_fork_exec(), platform_terminate_process()
Floating-Point Handling	platform_init_fpu()
File System Paths	platform_normalize_path(), platform_get_temp_dir()
Library Loading	platform_load_library(), platform_get_symbol()
Console Handling	platform_set_console_mode(), platform_get_console_size()