kselftests/arm64: add PAuth test for whether exec() changes keys

Kernel documentation states that it will change PAuth keys on exec() calls.

Verify that all keys are correctly switched to new ones.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Reviewed-by: Vincenzo Frascino <Vincenzo.Frascino@arm.com>
Reviewed-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200918104715.182310-4-boian4o1@gmail.com
Signed-off-by: Will Deacon <will@kernel.org>