Add globals and error codes

Added support for global variables.

This requires including `start.asm` as the first code. This includes
a `jmp` to 0x100 for user code, and a magic number that modules
can check to verify th the user has correctly reserved the space.

The magic number is 0xb301534c:

* This has the high bit set so is not a valid instruction

* On builds that ignore the high bit this decodes to `nor zr, zr,
  0x534c` a no-op.

* The immediate is "SL" for standardl ibrary.

Due to a bug start pads with U... 0, instead of @addr.

`start.asm` also initialises `sp` so it start just below the 64KiB LUT
memory alocation.

Added some error codes for the upcoming graphics module.

src/globals.asm

@@ -1,18 +1,28 @@
 ; Addresses of global variables
 
 ; If not double buffering these point to the same buffer
+; If double buffering they must the same size
 pub const FB_DISPLAY_PTR     = 16 ; U32 Currently displayed buffer
-pub const FB_DRAW_PTR        = 20 ; U32 Draw to this buffer
-pub const FRAME_BUFFER_SIZE  = 24 ; U32
-pub const PIXEL_WIDTH        = 28 ; U16
-pub const PIXEL_HEIGHT       = 30 ; U16
+pub const FB_DISPLAY_STRIDE  = 20 ; U32 Bytes in each row of the displayed buffer
+pub const FB_DRAW_PTR        = 24 ; U32 Draw to this buffer
+pub const FB_DRAW_STRIDE     = 28 ; U32 Bytes in each row of the draw buffer
+pub const FB_SIZE_BYTE       = 32 ; U32 In bytes
+pub const FB_WIDTH_PX        = 36 ; U16 Width of screen in pixels
+pub const FB_HEIGHT_PX       = 38 ; U16 Height of screen in pixels
 
 ; Log2 of width in pixels, e.g.
 ; * 10 => 1024 * 768
-pub const LOG_WIDTH          = 32 ; U8
+; *  8 =>  256 * 192
+pub const FB_LOG_WIDTH       = 40 ; U8
+pub const FB_LOG_STRIDE      = 41 ; U8 Log2 of FB_xxx_STRIDE in bytes
 
-pub const BYTES_PER_PIXEL    = 33 ; U8
+pub const FB_BYTES_PER_PIXEL = 42 ; U8 Specialisations should hardcode this
 ; Log2 of bytes per pixel
 ; * 0 => 8 bits per pixel
 ; * 2 => 32 bits per pixel
-pub const LOG_BPP            = 34 ; U8
+pub const FB_LOG_BPP         = 43 ; U8 Specialisations should hardcode this
+
+pub const MAGIC_ADDRESS_LONG = 12 ; U32 Location of the magic value
+pub const MAGIC_VALUE_LONG   = 0xb301534c ; U32 Full 32 bits of the magic value
+pub const MAGIC_ADDRESS      = 14 ; U16 Location of the low 16 bits of magic value
+pub const MAGIC_VALUE        = 0x534c ; U16 Low 16 bits of the magic value