cpu/x86/mp_init.c: Copy the stub parameter start32_offset into ramstage

Keep a copy of start32_offset into ramstage to avoid needing to pass
arguments, calling from assembly. Doing this in C code is better than
assembly.

Change-Id: Iac04358e377026f45293bbee03e30d792df407fd
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50765
Reviewed-by: Eugene Myers <cedarhouse1@comcast.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/cpu/x86/smm.h

@@ -60,8 +60,6 @@ struct smm_runtime {
 	u32 save_state_size;
 	u32 num_cpus;
 	u32 gnvs_ptr;
-	/* STM's 32bit entry into SMI handler */
-	u32 start32_offset;
 } __packed;
 
 struct smm_module_params {
@@ -88,6 +86,8 @@ struct smm_stub_params {
 	 * contiguous like the 1:1 mapping it is up to the caller of the stub
 	 * loader to adjust this mapping. */
 	u8 apic_id_to_cpu[CONFIG_MAX_CPUS];
+	/* STM's 32bit entry into SMI handler */
+	u32 start32_offset;
 	struct smm_runtime runtime;
 } __packed;