um: Enable SMP support on x86

um: Enable SMP support on x86

Implement spinlock support for SMP on UML/x86, leveraging x86's
spinlock implementation. In addition, to support SMP on CPUs that
do not support CX8, some additional work is required. However,
considering that such CPUs are already very outdated, and x86 is
also removing support for them [1], let's enable SMP support only
on CPUs that support CX8.

[1] https://lore.kernel.org/lkml/20250515085708.2510123-1-mingo@kernel.org/

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20251027001815.1666872-9-tiwei.bie@linux.dev
Signed-off-by: Johannes Berg <johannes.berg@intel.com>