[flang] Implement byte-swapped external unformatted I/O in runtime

Add SetConvert() to the OPEN statement's runtime API.
Add ByteswapOption() to the main program's runtime API.
Check a $FORT_CONVERT environment variable, too, for
a swapping specifier.

Reviewed By: sscalpone

Differential Revision: https://reviews.llvm.org/D84284