include: mm: increase readahead buffer
lib/memcopy: use glibc version
the kernel's memcpy and memmove is very inefficient. But the glibc version is
quite fast, in some cases it is 10 times faster than the kernel version. So I
introduce some memory copy macros and functions of the glibc to improve the
kernel version's performance.

The strategy of the memory functions is:
1. Copy bytes until the destination pointer is aligned.
2. Copy words in unrolled loops.  If the source and destination are not
   aligned in the same way, use word memory operations, but shift and merge
   two read words before writing.
3. Copy the few remaining bytes.

Signed-off-by: Miao Xie <miaox*******>
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*******>
update defconfig (for testing)