lib/string: use glibc version

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

Signed-off-by: Miao Xie <miaox*******>
Signed-off-by: zwliew <zhaoweiliew@gmail.com>

Conflicts:
	lib/string.c

Change-Id: Iac1ca83b5f41af1fca05a0948d5037de899d9799