[cx16] Enhanced CX16 support

[mlund]

This PR brings non-breaking changes to the Commander X16 target. Apologies for the combo of changes; let me know if I should split to more focused parts.

• Bug fix: leaf attributes accidentally inside function args.
• Bug fix: Function get_ostype() did not work.
• Bug fix: Function vpoke() had an endianness bug.
• Convert several KERNAL wrapper functions to inline asm, saving around 10-20 bytes per function.
  - cx16_k_screen_set_charset()
  - vpeek()
  - vpoke()
  - waitsync()
• Add unique assembler section names and progbits to all cbm_k_ functions.
• Removed superfluous ldx #0 before returning unsigned char (leftover from cc65).
• Add explicit enum variant type.
• Add missing emulator registers.
• Minor changes to code comments.
• Add labels for ROM banks as specified in the cx16-docs.
• Add struct for VERA PSG audio channels.
• Add assembler macro for JSRFAR access (KERNAL routine).
• Add volatile access to KERNAL_VERSION register.
• Use typedef struct {...} instead of struct {} for JoyState. This is better practice and implicit in C++.
• Add CBM_LOAD_* enum for controlling KERNAL function LOAD. CX16 has extended functionality.
• Compile-time size checks of some structs using static_assert. Guarded for C23/C++11 or higher.

[mysterymath]

This is a bit much for one PR; it'll take me a long time to review everything, which will block the easier stuff from getting in. (It's really nice to see a simple change in a PR and have it be so obviously correct that I can immediately just hit the button. Even if I need to do it 20 times, that's still preferable.)

Would you mind:

• Splitting out a PR for any NFC changes (cosmetics, code cleanliness refactoring, comments, etc)
• Splitting the other changes out by "topic"