Provide linker-configured PAL_BUF for neslib/nesdoug

[mysterymath]

Currently, the palette buffer used by neslib/nesdoug is hard-coded to the low portion of the hardware stack. This should be made a weak symbol instead to allow for the user to place it elsewhere. Stack is particularly valuable on the NES, since it enables techniques like pop-slide.

[cogwheel]

Would the same hold for VRAM_BUF and OAM_BUF? E.g. to be able to keep them all in RAM instead of PRG-RAM

[mysterymath]

Yep, that makes sense too.

[cogwheel]

Would it make sense to change the default (weak) PPU_BUF to be a normal array that doesn't share address space with anything else? This way overriding it to overlap the stack would be an opt-inmization. (#217 helps since you only pay for the space if you use the feature)

[mysterymath]

Hmm, I think yes to this one too. As always, it's this way because that's what I copied over during the nesdoug tutorial port, but using the hard stack for something should really be opt-in, even if there's e.g. a C macro to configure this. That's also a roughly backwards-compatible change; you shouldn't depend on the precise placement of library internal symbols.