fix two big bugs and add some code for arm920t mmu

there are two bugs which block the develop for two
weeks, finally find that it need to flush the cache
when switch process. a good news for the next development

Signed-off-by: Le Min <lemin9538@gmail.com>
implement the syscall fork and optimize memcpy

implement syscall fork and optimize memcpy using
asm code

Signed-off-by: Le Min <lemin9538@gmail.com>