Import @davidgiven's support for new targets

[johnwbyrd]

It seems that @davidgiven has already gone to the trouble to create llvm-mos linker scripts for half a dozen platforms we don't currently support. If we asked him nicely, I bet he would let us modify and repurpose those scripts for llvm-mos-sdk:

https://github.com/davidgiven/cpm65/tree/master/src/arch

This seems like a low-effort, high-impact way of supporting for 5 or 6 more platforms.

[asiekierka]

• [ ] apple2e
• [x] atari800 => We already have a port.
• [ ] bbcmicro
• [x] commodore => We already have a port.
• [x] neo6502 => https://github.com/llvm-mos/llvm-mos-sdk/pull/323 (from-scratch implementation)
• [ ] oric
• [ ] sorbus
• [x] x16 => We already have a port.