kselftest: arm64: Check access to GCS after mprotect(PROT_NONE)

kselftest: arm64: Check access to GCS after mprotect(PROT_NONE)

A GCS mapping should not be accessible after mprotect(PROT_NONE). Add a
kselftest for this scenario.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>