[flang] Add fixes and missing stmts to I/O runtime API handlers (ext. I/O part 8)

Complete the rework of the initial implementation of external I/O
to fix problems found in testing (tests to follow); add handlers
for hitherto unimplemented PAUSE, FLUSH, ENDFILE, BACKSPACE, and
REWIND statements.

Reviewed By: tskeith, sscalpone

Differential Revision: https://reviews.llvm.org/D83162