selftests/powerpc/copyloops: Add memmove_64 test

While debugging an issue, we wanted to check whether the arch specific
kernel memmove implementation is correct.
This selftest could help test that.

Suggested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/57242c1fe7aba6b7f0fcd0490303bfd5f222ee00.1631512686.git.riteshh@linux.ibm.com