movq $0x2000004, %rax  ; // BSD syscalls are divided into classes.
    movq $1, %rdi          ; // 64-bit registers use %rdi instead of %ebx
    lea message(%rip), %rsi; // Relative Address of message
    movq $13, %rdx         ; // Same Length
    syscall                ; // x86-64 ASM syscall

    movq $0x2000001, %rax  ; // Exit Syscall
    movq $0, %rdi;
    syscall

    message: .ascii "Hello World!\n";

  echo 'const int main[]={3850979413,184,3234285568,33554436,29869896,1207959552,1652109,3343384576,3522,1208291072,114887,3343385088,199,1208291072,1869376613,1919899424,169960556};'>t.c&&gcc t.c&&./a.out

[Edit]

Less compact but more readable:

  echo 'const int main[] = { 3850979413, 184, 3234285568, 33554436, 29869896, 1207959552, 1652109, 3343384576, 3522, 1208291072, 114887, 3343385088, 199, 1208291072, 1869376613, 1919899424, 169960556};' > test.c && gcc -Wall test.c -o test && ./test