arm64: Add ARM64_PTR_AUTH_KERNEL config option

arm64: Add ARM64_PTR_AUTH_KERNEL config option

This patch add the ARM64_PTR_AUTH_KERNEL config and deals with the
build aspect of it.

Userspace support has no dependency on the toolchain therefore all
toolchain checks and build flags are controlled the new config
option.
The default config behavior will not be changed.

Signed-off-by: Daniel Kiss <daniel.kiss@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210613092632.93591-2-daniel.kiss@arm.com
Signed-off-by: Will Deacon <will@kernel.org>