x86 SMM: Fix use with RELOCATABLE_RAMSTAGE

The value for _size was not evaluated correctly if ramstage
is relocated, make the calculation runtime.

While touching it, move symbol declarations to header file.

Change-Id: I4402315945771acf1c86a81cac6d43f1fe99a2a2
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/17784
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/cpu/x86/smm/smm_module_loader.c

@@ -45,9 +45,6 @@ struct smm_stub_params {
  */
 extern unsigned char _binary_smmstub_start[];
 
-/* This is the SMM handler that the stub calls. It is encoded as an rmodule. */
-extern unsigned char _binary_smm_start[];
-
 /* Per CPU minimum stack size. */
 #define SMM_MINIMUM_STACK_SIZE 32