Include HAL headers for custom chips

[mlund]

The MEGA65 project doesn't seem to provide any structured HAL headers for interacting with custom chips like VIC-IV, the hypervisor etc. KickC does include good headers, but these are not directly transferrable to CC65 and LLVM. There are also headers generated from iomap.txt that simply create defines for all symbols (flat).

Although this is not strictly "libc" stuff, it seems meaningful to include such headers here as most C/C++/Rust based project would benefit from both. My idea is to re-use C64 headers (sid, vic2) from CC65 and create a similar-styled HAL based on the current KickC headers. Ping @lydon42.