Export all labels to debug symbols

[kemenaran]

This PR enables the rgbasm -E option, which exports all labels to debug symbols, including unreferenced and local labels.

This allows us to see local labels in our debug tools. For instance, in the following code:

RenderLoop::
    ; Set DidRenderFrame
    ld   a, 1
    ldh  [hDidRenderFrame], a

.applyRegularScrollYOffset
    ; Compose the base offset and the screen shake offset
    ld   hl, wScreenShakeVertical
    ldh  a, [hBaseScrollY]
    add  a, [hl]

.setScrollY
    ld   [rSCY], a ; scrollY

The RenderLoop label (which is global) would be exported and visible in our debugger—but applyRegularScrollYOffset (which is a local label) wouldn't show.

Note about rgbds version

This option requires at least rgbds v0.2.5.