arch/x86: Allow SoC to run preparatory steps on shadow stack

Some SoCs may need to do some preparatory work before changing the
current shadow stack pointer (and thus, currently used shadow stack).
This patch adds a way for that, shielded by a Kconfig
(CONFIG_X86_CET_SOC_PREPARE_SHADOW_STACK_SWITCH).

As currently only 32 bit SoC may use this, support is only added to the
32 bit code.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>