[mega65] Add DMA hardware registers and DMA examples

[mlund]

The following adds support for the DMA controller on the MEGA65.

• Add _dmagic.h C header
• Add volatile MMIO access to F018 "DMAgic" controller
• Add C++ support library for constructing arbitrary DMA jobs as well as fill and copy functions.
• Add minimal C++ example with DMA fill and copy on screen. There is still an issue with DMA fill/copy routines likely due to code being optimized away. Will try to solve this on Discord.
• Add C++ example of vertical raster bars using the DMA hold feature as described in the MEGA65 Book. Below is a screenshot of the effect.
  
• Add DMA audio example for playing digitized sound samples. Preview:
  https://github.com/user-attachments/assets/8b6207fb-d81e-4daa-86e5-a050f1df026e

[mlund]

Here an initializer list would have been nice, but it doesn't seem to be supported by the compiler. That is dma.options = {DST_ADDR_BITS_OPT, 0xff};

[mysterymath]

Prefer include guards for nit-picky compiler-person reasons.

[mlund]

fixed.