disable vp9_iht8x8_64_add_neon

cherry picked from:
commit 0685ec767ce641baaf3ac8d5e24b1e68ab1b110b
Author: James Zern <jzern@google.com>
Date:   Sat Mar 3 12:47:24 2018 -0800

this causes test vector failures

BUG=webm:1403

Change-Id: I7d37a05fbf4641ea352c947053aa4eaeb7f5c318
3 files changed