cc65 is a complete cross development package for 65(C)02 systems, including a powerful macro assembler, a C compiler, linker, librarian and several other tools.
cc65 has C and runtime library support for many of the old 6502 machines, including
The libraries are fairly portable, so creating a version for other 6502s shouldn't be too much work.
If you want to contribute please get in touch with me.