lib: improve the performance of memcpy and memmove of the general version

the performance of memcpy and memmove of the general version is
very inefficient, this patch improved them.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Grarak <williye97@gmail.com>

Conflicts:
	lib/string.c

Conflicts:
	lib/string.c