Make MEM_ACCESS configurable

Make MEM_ACCESS configurable

Select MEM_ACCESS_ALWAYS_ON on x86 to mark that MEM_ACCESS is not
configurable on x86. Avoid selecting it on ARM.
Rename HAS_MEM_ACCESS to MEM_ACCESS everywhere. Add a prompt and a
description to MEM_ACCESS in xen/common/Kconfig.

The result is that the user-visible option is MEM_ACCESS, and it is
configurable only on ARM (disabled by default). At the moment the
arch-specific mem_access code remains enabled on ARM, even with
MEM_ACCESS=y.

The purpose is to reduce code size. The option doesn't depend on EXPERT
because it would be nice to ecurity-support configurations without
MEM_ACCESS and a non-expert should be able to disable it.

Suggested-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
CC: dgdegra@tycho.nsa.gov
CC: andrew.cooper3@citrix.com
CC: George.Dunlap@eu.citrix.com
CC: ian.jackson@eu.citrix.com
CC: jbeulich@suse.com
CC: julien.grall@arm.com
CC: konrad.wilk@oracle.com
CC: sstabellini@kernel.org
CC: tim@xen.org
CC: wei.liu2@citrix.com
---
Changes in v5:
- change MEM_ACCESS_ALWAYS_ON to bool
- change default for MEM_ACCESS, default y if MEM_ACCESS_ALWAYS_ON

Changes in v4:
- remove HAS_MEM_ACCESS
- move MEM_ACCESS_ALWAYS_ON to common
- combile default and bool to def_bool

Changes in v3:
- keep HAS_MEM_ACCESS to mark that an arch can do MEM_ACCESS
- introduce MEM_ACCESS_ALWAYS_ON
- the main MEM_ACCESS option is in xen/common/Kconfig

Changes in v2:
- patch added