[flang][openacc] Support fixed form sentinel format (#69970)

The OpenACC standard mentions directive format for fixed form source
files. The following sentinels are accepted:

```
!$acc directive-name [clause-list]
c$acc directive-name [clause-list]
*$acc directive-name [clause-list]
```

Update the parser to accepts these. A new option is added to `bbc` so
the change can be tested.