Add CFcommon.arch for arch-specific Kconfig options.

In accordance with [1], [2] and [3], move the x86-specific kernel option
CONFIG_HYPERVISOR_GUEST to CFcommon.i686 and CFcommon.x86_64, and also
move the x86/PowerPC CONFIG_KVM_GUEST Kconfig option to CFcommon.i686,
CFcommon.x86_64, and CFcommon.ppc64le.

The "arch" in CFcommon.arch is taken from the "uname -m" command.

[1] https://lore.kernel.org/all/20240427005626.1365935-1-zhouzhouyi@gmail.com/
[2] https://lore.kernel.org/all/059d36ce-6453-42be-a31e-895abd35d590@paulmck-laptop/
[3] https://lore.kernel.org/all/ZnBkHosMDhsh4H8g@J2N7QTR9R3/

Tested in x86_64 and PPC VM of Open Source Lab of Oregon State University.

Fixes: a6fda6da ("rcutorture: Tweak kvm options")
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>