[flang][runtime] Don't write implied ENDFILE for REC=/POS= (#79637)

An implied ENDFILE record, which truncates an external file, should be
written to a sequential unit whenever the file is repositioned for a
BACKSPACE or REWIND statement if a WRITE statement has executed since
the last OPEN/BACKSPACE/REWIND.

But the REC= and POS= positioning specifiers don't apply to sequential
units (they're for direct and stream units, resp.), so don't truncate
the file when they're used.