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>

idea is taken here - https://github.com/showp1984/android_kernel_lge_zee/commit/a72b1eb1c67ff9922a1c80e31d1d8e2b4aa798df