The PE32Header struct is only used by COFFYAML, for intermediate storage. The struct doesn't match the on-disk struct layout as it uses native integers instead of e.g. support::ulittle32_t, so just widen the fields to fit values for object::pe32plus_header, in addition to object::pe32_header.
This avoids truncating the 64 bit ImageBase for 64 bit executables.