x86, s390: Move ARCH_HAS_MEM_ENCRYPT definition to arch/Kconfig

powerpc is also going to use this feature, so put it in a generic location.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190806044919.10622-2-bauerman@linux.ibm.com