How about 31 bytes?

http://www.returninfinity.com/baremetal-helloworld.html

Why use ELF or Linux? Flat binaries (and a custom OS) to the rescue! :)

Why limit yourself to an OS? Design your own instruction set, with a single-bit 'hw' instruction!

As with all games (even Nomic) a game is only a game if there are rules that limit what one can do.

The esoteric language HQ9+ does this.

Indeed, except that one doesn't print "Hello world". :)

Oh, right, you got me, I totally didn't remember that, only the fact, that it was really small and I dug the fact that he put it inside the header.