Adjust ABI with unnamed function parameter

[fuhsnn]

The current way of ignoring unnamed parameters will create ABI mismatch, since chibicc implicitly assume parameter list is just how much local objects are created at function entry:

chibicc/parse.c

Line 3266 in 2c85b56

fn->params = locals;

We need dummy object to maintain the ABI info.

It can be observed by compiling this snippet:

int fn(int,int,int,int,int should_be_r8d) {
  return should_be_r8d;
}

  mov %edi, -12(%rbp)
  lea -12(%rbp), %rax
  movsxd (%rax), %rax
  jmp .L.return.fn
  ud2
.L.return.fn:
  mov %rbp, %rsp
  pop %rbp
  ret

%edi was used instead of %r8d