csky: Improve string operations performance by glibc code

These codes are copied from glibc-2.28/string directory, they are the
generic implementation for string operations. We may further optimize
them with assembly code in the future.

In fact these code isn't tested enough for kernel, but we've tested them
on glibc and it seems good. We just trust them :)

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>