Try the uC/51 (www.wickenhaeuser.com). It comes with a (s)printf() in C source (library), based on an universal printf()-formatter.
You may use the (intermediatelly) generated assembly... (but due to the compiler's optimiser,
you can even use (s)printf() on an AT89C2051,
so there is no need for assembly any more .. )

The DEMO is free and will produce 8kB code.
Jürgen