For more accurate compatibility with other compilers' extensions, accept
a bare exponent letter as valid real input to a formatted READ statement
only in a fixed-width input field. So it works with (G1.0) editing, but
not (G)/(D)/(E)/(F) or list-directed input.
Fixes https://github.com/llvm/llvm-project/issues/151465.
effa35d240
