Add a way to force two sections to be in the same bank

As discussed here: https://github.com/rednex/rgbds/issues/133

This could either be done in source code or by using the linkerscript.
```
SECTION "Code", ROMX
...
SECTION "Data", ROMX, BANK["Code"]
```
This would allow to have sections with different alignments if needed.

Start by reading the linked issue #133 and tracing how SECTION declarations and linker scripts assign sections to banks. The work is done when two sections can be forced into the same bank while retaining different alignments, using either source syntax or the linker script.

Written by the indexing model from the issue text.