x86, kaiser: add Kconfig

PARAVIRT generally requires that the kernel not manage its own page
tables.  It also means that the hypervisor and kernel must agree
wholeheartedly about what format the page tables are in and what
they contain.  KAISER, unfortunately, changes the rules and they
can not be used together.

I've seen conflicting feedback from maintainers lately about whether
they want the Kconfig magic to go first or last in a patch series.
It's going last here because the partially-applied series leads to
kernels that can not boot in a bunch of cases.  I did a run through
the entire series with CONFIG_KAISER=y to look for build errors,

Note from Hugh Dickins on why it depends on SMP:

	It is absurd that KAISER should depend on SMP, but
	apparently nobody has tried a UP build before: which
	breaks on implicit declaration of function
	'per_cpu_offset' in arch/x86/mm/kaiser.c.

	Now, you would expect that to be trivially fixed up; but
	looking at the System.map when that block is #ifdef'ed
	out of kaiser_init(), I see that in a UP build
	__per_cpu_user_mapped_end is precisely at
	__per_cpu_user_mapped_start, and the items carefully
	gathered into that section for user-mapping on SMP,
	dispersed elsewhere on UP.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
Cc: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
Cc: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: x86@kernel.org
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