If I'm remembering correctly there are some CL compilers that aim for runtime-free small native executables, even with realtime constraints, but I forget which.

Not CL, but there is ulisp (http://www.ulisp.com/) for microcontrollers, supposed to be really tiny, and there is Carp (https://github.com/carp-lang/Carp) which is without a GC so seems suitable for real-time stuff.