Non symex module should not use symex magic string SYMEX_DYNAMIC

Non-symex modules should not use the magic string used by the internals of goto-symex. This was discussed in #3481. The string was factored into `SYMEX_DYNAMIC_PREFIX` and put in util for now. 

- ansi-c / expr2c.cpp -> convert_symbol
- goto-instrument / race_check -> is_shared
- util / simplify_expr_pointer -> simplify_dynamic_object 