sim: h8300: simplify h8300_reg_{fetch,store} funcs

sim: h8300: simplify h8300_reg_{fetch,store} funcs

We can leverage the cpu->regs array rather than going through the
function helpers to get nice compact code.

Further, fix up the return values: return -1 when we can't find a
register (and let the caller write out warnings), return 2/4 when
we actually write out that amount, and handle the zero reg.