kselftest/arm64: Add simple test for MTE prctl

The current tests use the prctls for various things but there's no
coverage of the edges of the interface so add some basics. This isn't
hugely useful as it is (it originally had some coverage for the
combinations with asymmetric mode but we removed the prctl() for that)
but it might be a helpful starting point for future work, for example
covering error handling.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220419103243.24774-5-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>