Changed CPU cycles period tweak so it only affects the 486's;

Applied Laser XT ROM BASIC and Laser XT EMS patches from Greatpsycho.

src/mem.h

@@ -109,6 +109,7 @@ void mem_write_nulll(uint32_t addr, uint32_t val, void *p);
 
 mem_mapping_t bios_mapping[8];
 mem_mapping_t bios_high_mapping[8];
+mem_mapping_t romext_mapping;
 
 extern mem_mapping_t ram_high_mapping;