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/include/cpu/x86/smm.h

@@ -485,6 +485,10 @@ void mainboard_smi_sleep(u8 slp_typ);
 void smi_release_lock(void);
 #endif
 
+/* This is the SMM handler. */
+extern unsigned char _binary_smm_start[];
+extern unsigned char _binary_smm_end[];
+
 /* Get PMBASE address */
 u16 smm_get_pmbase(void);