[flang] Fix SELECTED_CHAR_KIND('DEFAULT') folding issue

Fix issue flang-compiler/f18#676
The issue was that the 'DEFAULT' case was handled
directly in fold.cc which did not lowercase/trim trailing
space of the argument befaore comparing to "default".

Modify the `Selected_char_kind` function to accept the default
char kind as argument and to return it if the processed argument
matches "default".

Original-commit: flang-compiler/f18@14222ae9148af4f99f59e6f9f0b928baa2da0b10
Reviewed-on: https://github.com/flang-compiler/f18/pull/679