[nasm] Leading underscores in labels not accepted

```asm
_Z1fILf7fc00000EEvv:
        ret
_Z1fILf7fc00001EEvv:
        ret
```
```html
<h- data-h=err>_</h-><h- data-h=name_labl_decl>Z1fILf7fc00000EEvv:</h->
        <h- data-h=name_inst>ret</h->
<h- data-h=err>_</h-><h- data-h=name_labl_decl>Z1fILf7fc00001EEvv:</h->
        <h- data-h=name_inst>ret</h->
```