KVM: selftests: Enable mmu_stress_test on arm64

KVM: selftests: Enable mmu_stress_test on arm64

Enable the mmu_stress_test on arm64.  The intent was to enable the test
across all architectures when it was first added, but a few goofs made it
unrunnable on !x86.  Now that those goofs are fixed, at least for arm64,
enable the test.

Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Marc Zyngier <maz@kernel.org>
Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>