personally i wouldn't be using WDC's tools, they definitely show their age... but if you get them to work and they work fine for your needs then go ahead.
but i would recommend something like
Calypsi C, it's C99 instead of C89, has pretty good documentation, and is actively being maintained.
i also remember vbc (a forum member) talking about working on a VBCC port for the 65816, but that was a really long time ago and i wonder what happend to that project since (from my experience) making a crappy but functional backend shouldn't take that long, right?
anyways back to the WDC tools, i don't know how exactly they handle their standard libraries (but from the documentation i can see that they include them), but i'd assume that similar to newlib, the included libraries would only contain the hardware independent components which would make calls to system specific functions (open, close, read, write, sbrk, etc.)
to see if that's the case you could just try it out. make a simple hello world program with printf, include the given library while linking and then see which function the compiler will complain about being missing.
or if they include the source code for the used libraries (which they should) try to see if you can find any function stubs for read, write, etc. or grep (findstr on windows) through them to find any mentions of functions like that, which would also tell you what arguments they need.
then you just write them (either in assembly or C) assemble/compile them to object files and throw them into a library to link along side your program.