[flang][runtime] Emit leading spaces in NAMELIST output (#76846)

As NAMELIST output is a variant of list-directed output, its editing
must produce leading spaces on (most) output records to effect carriage
control. These spaces are required by the language standard and
implemented by nearly all other Fortran compilers (except GNU).

Fixes https://github.com/llvm/llvm-project/issues/76798.