innodb_log_file_mmap is not defined on 32-bit systems

otto pointed out that the configuration parameter innodb_log_file_mmap is not available on 32-bit target platforms, for no apparent reason.

When fixing MDEV-34062 I had thought that including two ib_logfile0 parsers would unnecessarily increase the size of the code. Technically, we can in fact memory-map the InnoDB write-ahead log ib_logfile0 also on 32-bit systems, provided that the size is small enough so that the virtual address space will not be exhausted.