@Suiseiseki

> If GNUboot doesn't init the hardware and then launch the GNU GRUB OS on my computer and then launch GNU Linux-libre and then launch the init and then launch the rest of the GNU and other software, nothing happens.

Yes, the bootloader is important. (But you should use a smaller one.)

> In this case, printf() is a function implemented by glibc, not Linux.

It has always been in libc rather than the kernel.

> If you try to pass a proprietary binary compiled for a Unix to Linux via a loader, Linux won't be able to make any sense of it,

I don't have a PDP-11 to test on.

> as it does not re-implement *any* Unix interface

See linux-0.11.tar.gz's include/unistd.h. From the beginning, it was pretending to be Unix by implementing those interfaces. Some of that has been moved outside the kernel, but as I said, Linux has stopped pretending to be Unix.