Support assembler sources in ca65 format

[johnwbyrd]

When I built the assembler, I left hooks in for supporting assembler input in other formats, including xa65 and ca65. (I put in an AsmParserVariant for ca65.) It would be helpful for people coming from ca65, to support assembler sources that use ca65 idioms, macros, and functions.

Now, the ca65 assembler specification is huge. I don't think we will want, or need, to implement every little ca65 feature in our assembler. But a more reasonable target would be to start by supporting sufficient ca65 functionality to be able to compile the ca65 libraries, and have them match 1:1 with the code compiled by ca65.

See also

llvm-mos/llvm/lib/Target/MOS/MOS.td

Line 52 in 2550bb7

def MOSAsmParserVariantCA65 : AsmParserVariant {

.