sim: h8300: drop separate eightbit memory buffer

The h8300 sim has its own implementation for memory handling that I'd
like to replace with the common sim memory code.  However, it's got a
weird bit of code it calls "eightbit mem" that makes this not as easy
as it would otherwise be.  The code has this comment:
/* These define the size of main memory for the simulator.

   Note the size of main memory for the H8/300H is only 256k.  Keeping it
   small makes the simulator run much faster and consume less memory.

   The linker knows about the limited size of the simulator's main memory
   on the H8/300H (via the h8300h.sc linker script).  So if you change
   H8300H_MSIZE, be sure to fix the linker script too.

   Also note that there's a separate "eightbit" area aside from main
   memory.  For simplicity, the simulator assumes any data memory reference
   outside of main memory refers to the eightbit area (in theory, this
   can only happen when simulating H8/300H programs).  We make no attempt
   to catch overlapping addresses, wrapped addresses, etc etc.  */

I've read the H8/300 Programming Manual and the H8/300H Software Manual
and can't find documentation on it.  The closest I can find is the bits
about the exception vectors, but that sounds like a convention where the
first 256 bytes of memory are used for a special purpose.  The sim will
actually allocate a sep memory buffer of 256 bytes and you address it by
accessing anywhere outside of main memory.  e.g. I would expect code to
access it like:
	uint32_t *data = (void *)0;
	data[0] = reset_exception_vector;
not like the sim expects like:
	uint8_t *data = (void *)0x1000000;
	data[0] = ...;

The gcc manual has an "eightbit_data" attribute:
	Use this attribute on the H8/300, H8/300H, and H8S to indicate that
	the specified variable should be placed into the eight-bit data
	section. The compiler generates more efficient code for certain
	operations on data in the eight-bit data area. Note the eight-bit
	data area is limited to 256 bytes of data.

And the gcc code implies that it's accessed via special addressing:
   eightbit_data: This variable lives in the 8-bit data area and can
   be referenced with 8-bit absolute memory addresses.

I'm fairly certain these are referring to the 8-bit addressing modes
that allow access to 0xff00 - 0xffff with only an 8-bit immediate.
They aren't completely separate address spaces which this eightbit
memory buffer occupies.

But the sim doesn't access its eightbit memory based on specific insns,
it does it purely on the addresses requested.

Unfortunately, much of this code was authored by Michael Snyder, so I
can't ask him :(.  I asked Renesas support and they didn't know:
https://renesasrulz.com/the_vault/f/archive-forum/6952/question-about-eightbit-memory

So I've come to the conclusion that this was a little sim-specific hack
done for <some convenience> and has no relation to real hardware.  And
as such, let's drop it until someone notices and can provide a reason
for why we need to support it.

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