new target: Atari 5200 console

This PR contains support for a new target, the Atari 5200 console.

Although the 5200 is quite similar to the Atari computer, it's still too different to make the new target a subtarget of atari (like it is done with atarixl). The 5200 has the same custom chips as the Atari computers, but they are at different memory locations. And the ROM of the 5200 is very basic and provides almost none of the services the computer ROM provides. The Atari XL ROM is 16K, while the 5200 ROM is 2K (and 1K of it is used by the character generator). The joysticks are different, too.

When compiling for this target a __ATARI5200__ macro is predefined.

I successfully compiled and run a modified version of the joy-test.c program. The modifications were due to the default screen size of 20x24 and the calls to the oserror functions (which are not supported by this target).