[request] implement RETRO_ENVIRONMENT_SET_MEMORY_MAPS for SMS (and other systems)

[beauxq]

In RetroArch, I've been using the network commands READ_CORE_RAM and WRITE_CORE_RAM for something that they're not intended for.
And the people at RetroArch tell me I should be using READ_CORE_MEMORY and WRITE_CORE_MEMORY instead.
But I can't use those if the emulator doesn't implement RETRO_ENVIRONMENT_SET_MEMORY_MAPS

From this code, it looks like it's implemented for SYSTEM_MCD, but not for SMS or other systems.
But I'm not sure whether I'm understanding it correctly.

Genesis-Plus-GX/libretro/libretro.c

Lines 2602 to 2620 in 667158b

	static void set_memory_maps(void)
	{
	if (system_hw == SYSTEM_MCD)
	{
	const size_t SCD_BIT = 1ULL << 31ULL;
	const uint64_t mem = RETRO_MEMDESC_SYSTEM_RAM;
	struct retro_memory_map mmaps;
	struct retro_memory_descriptor descs[] = {
	{ mem, work_ram, 0, 0xFF0000, 0, 0, 0x10000, "68KRAM" },
	/* virtual address using SCD_BIT so all 512M of prg_ram can be accessed */
	/* at address $80020000 */
	{ mem, scd.prg_ram, 0, SCD_BIT | 0x020000, 0, 0, 0x80000, "PRGRAM" },
	};
	mmaps.descriptors = descs;
	mmaps.num_descriptors = sizeof(descs) / sizeof(descs[0]);
	environ_cb(RETRO_ENVIRONMENT_SET_MEMORY_MAPS, &mmaps);
	}
	}

It would be nice if I could use READ_CORE_MEMORY and WRITE_CORE_MEMORY with Genesis-Plus-GX (and SMS-Plus-GX), with SMS games.