[flang][runtime] Allow OPEN(n,ENCODING=) to change the encoding

OPEN statements can be used to change some, but not all, attributes
of units that have already been opened.  The I/O runtime library
wasn't allowing ENCODING= to be changed.  Every other Fortran compiler
permits this usage, and it's safe and useful, so allow it.
(Otherwise there's no good way to ensure that the preconnected
unit 6 is in UTF-8 mode.)

Differential Revision: https://reviews.llvm.org/D154379