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

libsrc/atmos/joy/atmos-pase.s

@@ -28,17 +28,6 @@
 
         .addr   $0000
 
-; Button state masks (8 values)
-
-        .byte   $10                     ; JOY_UP
-        .byte   $08                     ; JOY_DOWN
-        .byte   $01                     ; JOY_LEFT
-        .byte   $02                     ; JOY_RIGHT
-        .byte   $20                     ; JOY_FIRE
-        .byte   $00                     ; Future expansion
-        .byte   $00                     ; Future expansion
-        .byte   $00                     ; Future expansion
-
 ; Jump table.
 
         .addr   INSTALL