Add missing arm64-neon function declaration. I'm trying to get to where we can turn on -Werror=implicit-function-declaration for LP64, so we can avoid bugs where we drop the top half of a pointer because the compiler's assuming 'int'. (cherry picked from commit dd333a75e52c117f563bbf3449dffabda2fbcdcf) Change-Id: I1ed7ef57abc85209fad9978a44e629945ff08338