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/southbridge/intel/i82801dx/smi.c

@@ -25,9 +25,6 @@
 #include <string.h>
 #include "i82801dx.h"
 
-extern unsigned char _binary_smm_start;
-extern unsigned char _binary_smm_size;
-
 /* I945 */
 #define SMRAM		0x90
 #define   D_OPEN	(1 << 6)
@@ -321,7 +318,8 @@ static void smm_install(void)
 				D_OPEN | G_SMRAME | C_BASE_SEG);
 
 	/* copy the real SMM handler */
-	memcpy((void *)0xa0000, &_binary_smm_start, (size_t)&_binary_smm_size);
+	memcpy((void *)0xa0000, _binary_smm_start,
+		_binary_smm_end - _binary_smm_start);
 	wbinvd();
 
 	/* close the SMM memory window and enable normal SMM */