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: faux123 <reioux@gmail.com>
Signed-off-by: Pafcholini <pafcholini@gmail.com>