That was not the solution.
The linking toolchain (gcc) is using newlib, but the __aeabi_memclr from that newlib is not the one that is inserted into the output executable.
Rustc ships with compiler-rt , a LLVM project. This project also contains implementations for __aeabi_memfoo.
So is this a bug in rustc? Tips still appreciated 
-Daniel (Sorry for the monologue)