_dl_fini: Remove internal_function attribute

Assembler code passes the address of _dl_fini to __libc_start_main,
whose function pointer argument lacks the attribute.  This means
that calls could use the wrong ABI.  Fortunately, for zero-parameter
void-returning functions, internal_function does not change ABI
on i386 (the only architecture which uses internal_function), so
this inconsistency was harmless (which is why it had not been
noticed so far).