[flang][runtime] Improve Fortran I/O behavior when main program is C/C++

Ensure that I/O buffers are flushed at the end of the program,
and properly detect ttys for predefined units so that Fortran
output is interspersed with output from non-Fortran code.

Fixes https://github.com/llvm/llvm-project/issues/56944

Differential Revision: https://reviews.llvm.org/D132152

Update: fix typo in comment