[flang] Support legacy usage of 'A' edit descriptors for integer & real

The 'A' edit descriptor once served as a form of raw I/O of bytes
to/from variables that weren't of type CHARACTER (which itself
didn't exist until F'77).  This usage was especially common for
output of numeric variables that had been initialized with Hollerith.

Differential Revision: https://reviews.llvm.org/D112346