What version of Arduino IDE and what version of avr-gcc are you using with core13?
Have you tried compiling with MicroCore with and without LTO enabled?
Since you're not inializing using void setup() and void loop(), none of the "Arduino functionality" is initialized. My therory is that this may be caused by the compiler or avr-libc, not the MicroCore source code (because you don't make use of it).