MCVE for ARM mode ODR violation

a.cpp

#include "a.hpp"

A::~A() { }

a.hpp

#pragma once

class A {
public:
  A();
  virtual ~A();
};

build.log

arm-none-eabi-gcc -x c++ -o main.o -c -Wall -flto -ffunction-sections -mthumb-interwork -mcpu=cortex-a9 -O2 -mfpu=neon -marm main.cpp
arm-none-eabi-gcc -x c++ -o a.o -c -Wall -flto -ffunction-sections -mthumb-interwork -mcpu=cortex-a9 -O2 -mfpu=neon -mthumb a.cpp
arm-none-eabi-gcc -o main --specs=nano.specs --specs=rdimon.specs -Wall -flto -ffunction-sections -mthumb-interwork -mcpu=cortex-a9 -O2 -mfpu=neon main.o a.o
a.hpp:5:7: warning: type 'struct A' violates the C++ One Definition Rule [-Wodr]
    5 | class A {
      |       ^
a.hpp:5:7: note: a type with different virtual table pointers is defined in another translation unit
    5 | class A {
      |       ^

main.cpp

#include "a.hpp"

int main(int argc, char const * const * argv) {
  A a;
}

Makefile

CFLAGS=-Wall -flto -ffunction-sections -mthumb-interwork -mcpu=cortex-a9 -O2 -mfpu=neon

main: main.o a.o
	arm-none-eabi-gcc -o $@ --specs=nano.specs --specs=rdimon.specs $(CFLAGS) $^

a.o: a.cpp
	arm-none-eabi-gcc -x c++ -o $@ -c $(CFLAGS) -mthumb $<

main.o: main.cpp
	arm-none-eabi-gcc -x c++ -o $@ -c $(CFLAGS) -marm $<

.PHONY: clean
clean:
	rm -f *.o
	rm -f main