[flang] Lower c_ptr_eq/ne for iso_c_binding (#85135)

Comparing c_ptr type for equality or inequality is raising an error. 

```
not yet implemented: intrinsic module procedure: c_ptr_eq
```
or this one for inequality
```
not yet implemented: intrinsic module procedure: c_ptr_ne
```

This patch adds a lowering for them and fix the `__fortran_builtins.f90`
module for inequality.