[OpenMP] Add Fortran support for omp_* functions needing adapters

Taking omp_get_uid_from_device and omp_get_device_from_uid as examples.

OpenMP 6.0 defines these functions as follows:
```
const char *omp_get_uid_from_device(int device_num);

character(:) function omp_get_uid_from_device(device_num)
  pointer :: omp_get_uid_from_device
  integer, intent(in) :: device_num
```
and
```
int omp_get_device_from_uid(const char *uid);

integer function omp_get_device_from_uid(uid)
  character(len=*), intent(in) :: uid
```

As a result, we cannot directly map the Fortran API declarations to the
C API declarations. We need some adapter code to do the conversion.

However, this means that we cannot just define the two functions as
omp_* in kmp_ftn_entry.h without clashing with the adapter code.

The current situation is less than ideal, as demonstrated by this draft
PR, which shall serve as a base for discussing different solution
approaches.