> the compiler assumes that the inline asm can fall through and prints a "function declared 'noreturn' should not return" warning.

It actually can (Hint: what happens if the operating system `iret`s from its int 3 handler?), although it's probably not a issue in practice. Regardless, you don't need __builtin_unreachable to write:

  void myabort(void) __attribute__((noreturn));
  void myabort(void) {
    asm("int3");
    myabort(); // might need `return myabort();` to force TCO,
    // but gcc doesn't like that and it should work anyway
    }
  # Assuming tail-call optimization etcetera, this produces:
  myabort:
    int3
    jmp myabort

which is a correct implementation.

However, if you're implementing built-in/standard functions like abort, you presumably know what compiler you're using and don't need a std interface in the first place. There's zero legitimate reason to use a undefined-behaviour-based `unreachable` in application code.