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/start.asm

@@ -1,4 +1,19 @@
-@0 ; Reserve space for globals
- 
-jmp 0x100 ; jump over them
-@0x100 ; start of user code
+;@0 ; Reserve space for globals
+
+; First initialise the stack pointer
+nor sp, zr, 0xffff
+; Jump over the globals to user code
+jmp 0x100
+
+U32 0 ; 4 bytes
+
+; MAGIC_VALUE
+;@0x0c
+U32 0xb301534c
+
+; Pad with zeroes since the @addr feature is currently broken.
+; * https://discord.com/channels/828292123936948244/1545010596246847568
+;
+U1920 0  ; 240 bytes
+
+;@0x100 ; start of user code