[mega65] Add DMA hardware registers and DMA examples

[mlund]

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

• [x] Add _dmagic.h C header
• [x] Add volatile MMIO access to F018 "DMAgic" controller
• [x] Add C++ support library for constructing arbitrary DMA jobs as well as fill and copy functions.
• [x] 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.
• [x] 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.
• [x] Add DMA audio example for playing digitized sound samples. Preview: https://github.com/user-attachments/assets/8b6207fb-d81e-4daa-86e5-a050f1df026e