[flang] Respect BZ mode in exponent parts, too

The Fortran standard discusses BZ mode (treat blanks as zero digits)
explicitly in its effect on the editing of the digits prior to the
exponent part, but doesn't mention it in description of the
exponent part.  Other compilers honor BZ mode in the exponent,
so we should do so too.  So "1 e 1 " is 1.E11 in BZ mode.

Differential Revision: https://reviews.llvm.org/D87653