[flang][runtime] Handle endian-swapped record headers & footers on input

The runtime I/O library correctly handles endianness conversions on payload
data I/O and on the output of sequential record headers and footers, but
does not swap endianness when required when reading sequential record headers
and footers back in for READ and BACKSPACE statements.  Mea culpa.  Fix.

Fixes https://github.com/llvm/llvm-project/issues/57126

Differential Revision: https://reviews.llvm.org/D132168