x86-64: Optimize memcmp-avx2-movbe.S for short difference

Check the first 32 bytes before checking size when size >= 32 bytes
to avoid unnecessary branch if the difference is in the first 32 bytes.
Replace vpmovmskb/subl/jnz with vptest/jnc.

On Haswell, the new version is as fast as the previous one.  On Skylake,
the new version is a little bit faster.

	* sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S (MEMCMP): Check
	the first 32 bytes before checking size when size >= 32 bytes.
	Replace vpmovmskb/subl/jnz with vptest/jnc.