Add "external" DATA statements

[dmsc]

In the cross-compiler, it could be useful to allow DATA statements without an actual definition, this will allow accessing external DATA as arrays or including binary files.

Example syntax:

DATA Image() byte asm
GR.8
MOVE ADR(Image), DPEEK(88), 40*192
GET K

And, in an assembly file:

  .export fb_IMAGE
  .data
fb_IMAGE:
  .incbin "myimage.raw"