[flang] Lower selected_char_kind to runtime call (#93095)

Runtime support has been added in
https://github.com/llvm/llvm-project/pull/89691. This patch adds
lowering in a similar way than `selected_int_kind`, `selected_real_kind`
and `selected_logical_kind` added in #93091.

Some gfortran tests can be enabled after this patch is landed.

- `Fortran/gfortran/regression/selected_char_kind_1.f90`
- `Fortran/gfortran/regression/selected_char_kind_4.f90`