[flang][runtime] Fix trailing blanks for Gw.dEe output editing (#75263)

When generalized numeric output editing of real data maps to Fw.d output
editing, either two or four trailing blanks are emitted depending on the
presence and value of 'e'. The code that detects field width overflow
didn't take these trailing blanks into account, and sometimes the field
width adjustment was producing an F0.d output edit descriptor (no fixed
field width). Fix by retaining the original field width, but requiring
it to also accommodate the trailing blanks.

Fixes llvm-test-suite/Fortran/gfortran/regression/fmt_g.f.