cpu/x86/mp_init.c: Generate a C header to get start32 offset

In the current design the relocatable parameters are used to know the
offset of the 32bit startpoint. This requires back and forward
interaction between the stub, the loader and the mp init code. This
makes the code hard to read.

This is static information known at buildtime, so a better way to deal
with this is to generate a header that contains this offset.

Change-Id: Ic01badd2af11a6e1dbc27c8e928916fedf104b5b
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64625
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/cpu/x86/smm.h

@@ -107,8 +107,6 @@ struct smm_stub_params {
 	 * contiguous like the 1:1 mapping it is up to the caller of the stub
 	 * loader to adjust this mapping. */
 	u16 apic_id_to_cpu[CONFIG_MAX_CPUS];
-	/* STM's 32bit entry into SMI handler */
-	u32 start32_offset;
 } __packed;
 
 /* smm_handler_t is called with arg of smm_module_params pointer. */