Xen only supports modern CPUs even when running a 32-bit kernel, and it now
requires a kernel built for a 64 byte (or larger) cache line:

In file included from <command-line>:
In function 'xen_vcpu_setup',
    inlined from 'xen_vcpu_setup_restore' at arch/x86/xen/enlighten.c:111:3,
    inlined from 'xen_vcpu_restore' at arch/x86/xen/enlighten.c:141:3:
include/linux/compiler_types.h:435:45: error: call to '__compiletime_assert_287' declared with attribute error: BUILD_BUG_ON failed: sizeof(*vcpup) > SMP_CACHE_BYTES
arch/x86/xen/enlighten.c:166:9: note: in expansion of macro 'BUILD_BUG_ON'
  166 |         BUILD_BUG_ON(sizeof(*vcpup) > SMP_CACHE_BYTES);
      |         ^~~~~~~~~~~~

Enforce the dependency with a whitelist of CPU configurations. In normal
distro kernels, CONFIG_X86_GENERIC is enabled, and this works fine. When this
is not set, still allow Xen to be built on kernels that target a 64-bit
capable CPU.

Fixes: db283230 ("x86/xen: fix percpu vcpu_info allocation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-by: Alyssa Ross <hi@alyssa.is>
Link: https://lore.kernel.org/r/20231204084722.3789473-1-arnd@kernel.orgSigned-off-by: Juergen Gross <jgross@suse.com>