sd->memory_size was only initialized in init_pointers(), that was
called before the executable was loaded and the machine-type detection
was run.  Even though sim_load() duplicated most of the code in
init_pointers(), it failed to update the memory_size in sd, so if we
attempted to run code from a memory location not available on the
original h8300, we'd get a confusing illegal-instruction error,
because h8_get_cache_idx would return -1.

This patch updates sim_load() such that it matches init_pointers() in
this regard.  Ok to install?