global variable always initialized zero

I've been writing an OS using this tutorial. I am at the part where the boot loader is completed and C is used for programming (and then linked together ...). But that just as a note, I believe the problem I have is related to gcc.

I build an i386-elf cross compiler for the OS. And everything works fine, I can execute my code everything works. Except that all global variables are initialized zero, although I provided a default value.

int test_var = 1234;

// yes, void main() is correct (the boot-loader will call this)
void main() {} 

If I debug this code with GDB, I get: (gcc-7.1.0, target: i328-elf)

(gdb) b main
Breakpoint 1 at 0x1554: file src/kernel/main.c, line 11.
(gdb) c
Continuing.

Breakpoint 1, main () at src/kernel/main.c:11
11  void main() {
(gdb) p test_var
$1 = 0

If i run the same code on my local machine (gcc-6.3.0, target: x86_64), it prints 1234.

My question is: Did I misconfigure gcc, is this a mistake in my OS, is this a known problem? I couldn't find anything about it.

My entire source-code: link I use the following commands to compile my stuff:

# ...
i386-elf-gcc -g -ffreestanding -Iinclude/ -c src/kernel/main.c -o out/kernel/main.o
# ...
i386-elf-ld -e 0x1000 -Ttext 0x1000 -o out/kernel.elf out/kernel_entry.o out/kernel/main.o # some other stuff ...
i386-elf-objcopy -O binary out/kernel.elf out/kernel.bin
cat out/boot.bin out/kernel.bin > out/os.bin
qemu-system-i386 -drive "format=raw,file=out/os.bin"

EDIT: As @EugeneSh. suggested here some logic to make sure, that it's not removed:

#include <cpu/types.h>
#include <cpu/isr.h>

#include <kernel/print.h>

#include <driver/vga.h>

int test_var = 1234;

void main() {
  vga_text_init();

  switch (test_var) {
    case 1234: print("That's correct"); break;
    case 0: print("It's zero"); break;

    // I don't have a method like atoi() in place, I would use
    // GDB to get the value
    default: print("It's something else");
  }
}

Sadly it prints It's zero