This would be a portable C (i.e. non x86-64, but still Linux) with just 2 syscalls:

  $ cat a.c
  #define _GNU_SOURCE
  #include <unistd.h>
  #include <sys/syscall.h>

  void _start(void) {
    syscall(__NR_write, 1, "Hello world!\n", 13);
    syscall(__NR_exit_group, 1);
  }

  $ gcc -nostartfiles a.c -o a -static
  $ ls -l a
  -rwxr-xr-x 1 user user 9584 jan  5 00:07 a
  $ strip -s a; ls -l a
  -rwxr-xr-x 1 user user 9000 jan  5 00:10 a
  $ strace ./a
  execve("./a", ["./a"], 0x7ffd4d4f8250 /* 67 vars */) = 0
  write(1, "Hello world!\n", 13)          = 13
  exit_group(1)                           = ?
  +++ exited with 1 +++

I'm not actually sure why syscall() is inlined and glibc code is not included here while compiling with -static, but well, it works. Maybe it's because syscall() is a macro, or maybe it's some kind of ld code minimization (removing unused/unreachable code) trick which seems to be common in modern static linkers (ld, gold, lld).