x86_64: Add memcmpeq.S to fix disable-multi-arch build

The following commit:

    commit cf4fd28e
    Author: Noah Goldstein <goldstein.w.n@gmail.com>
    Date:   Tue Oct 26 19:43:18 2021 -0500

Broke --disable-multi-arch build for x86_64 because x86_64/memcmpeq.S
was not defined outside of multiarch and the alias for __memcmpeq in
x86_64/memcmp.S was removed.

This commit fixes that issue by adding x86_64/memcmpeq.S.

make xcheck passes on x86_64 with and without --disable-multi-arch