KVM: selftests: Build the memslot tests for arm64

Add memslot_perf_test and memslot_modification_stress_test to the list
of aarch64 selftests.

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210907180957.609966-3-ricarkol@google.com