Кстати, насчет rtlib, вот это видел?: http://compiler-rt.llvm.org/
The current feature set of compiler-rt is:

Full support for the libgcc interfaces on supported targets.
High performance hand tuned implementations of commonly used functions like __floatundidf in assembly that are dramatically faster than the libgcc implementations.
A target-independent implementation of the Apple "Blocks" runtime interfaces.
Вроде оно в целом на C написано с вариациями оптимизированного кода на асме для разных процессоров. Т.е. по идее для начала можно тупо заюзать и для Z80, оптимизациями можно и позже заняться.

О libgcc:
http://gcc.gnu.org/onlinedocs/gccint/Libgcc.html
Most of the routines in libgcc handle arithmetic operations that the target processor cannot perform directly. This includes integer multiply and divide on some machines, and all floating-point and fixed-point operations on other machines. libgcc also includes routines for exception handling, and a handful of miscellaneous operations.
Т.е. софтовая реализация умножений, делений, fp-операций и т.п.