cpu/x86/mp_init: Allow stub sizes larger than the save state size

The permanent handler module argument 'save_state_size' now holds the
meaning of the real save state size which is then substracted from the
CPUs save state 'top' to get the save state base.

TESTED with qemu Q35 on x86_64 where the stub size exceeds the AMD64
save state size.

Change-Id: I55d7611a17b6d0a39aee1c56318539232a9bb781
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50770
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/cpu/x86/smm.h

@@ -138,6 +138,7 @@ struct smm_loader_params {
 	size_t per_cpu_stack_size;
 	size_t num_concurrent_stacks;
 
+	size_t real_cpu_save_state_size;
 	size_t per_cpu_save_state_size;
 	size_t num_concurrent_save_states;