Removed joy_masks array.

So far the joy_masks array allowed several joystick drivers for a single target to each have different joy_read return values. However this meant that every call to joy_read implied an additional joy_masks lookup to post-process the return value.

Given that almost all targets only come with a single joystick driver this seems an inappropriate overhead. Therefore now the target header files contain constants matching the return value of the joy_read of the joystick driver(s) on that target.

If there indeed are several joystick drivers for a single target they must agree on a common return value for joy_read. In some cases this was alredy the case as there's a "natural" return value for joy_read. However a few joystick drivers need to be adjusted. This may cause some overhead inside the driver. But that is for sure smaller than the overhead introduced by the joy_masks lookup before.

!!! ToDo !!!

The following three joystick drivers become broken with this commit and need to be adjusted:
- atrmj8.s
- c64-numpad.s
- vic20-stdjoy.s

include/em/em-kernel.h

@@ -52,6 +52,7 @@ typedef struct {
     /* Driver header */
     char                id[3];          /* Contains 0x65, 0x6d, 0x64 ("emd") */
     unsigned char       version;        /* Interface version */
+    void*                               /* Library reference */
 
     /* Jump vectors. Note that these are not C callable */
     void*               install;        /* INSTALL routine */