String.c fixes and added more ram as per Simon Docs

When using syscall memcpy function it turns out that is did not work all the time. I have replaced it with functions from dietlibc. I also noticed that the casio prizm has 512kb of stack not 64kb so I changed that. (This is mentioned in "Simon docs".). Also for memmove you don't need to malloc a copy of the data just copy it starting from the end of data if there is overlap.