[flang] Rename EXTERN_C_END to FORTRAN_EXTERN_C_END

Since EXTERN_C_END is a macro defined in Windows system headers

Original-commit: flang-compiler/f18@4c6e5608adddf25a53fbdbfd5f0e1b98278cc5ec
Reviewed-on: https://github.com/flang-compiler/f18/pull/1051