As other comments have noted, the asm statement needs to have its input/output registers specified to ensure the compiler doesn't erase the "unused" values.

Working example: https://john-millikin.com/unix-syscalls#linux-x86-64-gnu-c

Adapted to use main():

  static const int STDOUT = 1;
  static const int SYSCALL_WRITE = 1;
  static const char message[] = "Hello, world!\n";
  static const int message_len = sizeof(message);

  int main() {
   register int         rax __asm__ ("rax") = SYSCALL_WRITE;
   register int         rdi __asm__ ("rdi") = STDOUT;
   register const char *rsi __asm__ ("rsi") = message;
   register int         rdx __asm__ ("rdx") = message_len;
   __asm__ __volatile__ ("syscall"
    : "+r" (rax)
    : "r" (rax), "r" (rdi), "r" (rsi), "r" (rdx)
    : "rcx", "r11");
   return 0;
  }

Test with:

  $ gcc -o hello hello.c
  $ ./hello
  Hello, world!