Support global = global initializers

[lynn]

These aren't supported:

int x[2] = {1, 1};
int *y = x;

// or more commonly...
char *message = "hi";

The compiler will complain unsupported label+addend.

chibicc writes

// Only the following expressions are allowed in an initializer:
//
//  1. A constant such as a number or a string literal
//  2. An address of another global variable with an optional addend
//
// It is obvious that we can embed (1) to an object file as static data.
// (2) may not be obvious why that can result in static data, but
// the linker supports an expression consisting of a label address
// plus/minus an addend, so (2) is allowed.

But uxnasm doesn't really support (2) so I stubbed it out with this unsupported label+addend error message.

Instead what you'd need to do in uxnasm is put another label on the same data…

@x
@y 
    0001
    0001

@message
@.L.data.0
    68
    69

Which hurts the single-pass-y-ness a little bit. And if we want to support the "addend" thing, something like

int x[5] = {1, 2, 3, 4, 5};
int *y = x + 2;

would have to be

@x
    0001
    0002
@y  
    0003
    0004
    0005

which is even trickier.

[hikari-no-yume]

Instead what you'd need to do in uxnasm is put another label on the same data…

@x
@y

Hmm I don't think that's right, the value of y isn't the same as x but rather its value is the address of x. I think we can do this with the “raw absolute addressing rune”:

@x
  =y