cbfstool: add a constant for the aarch64

Change-Id: Ide2c8b778447de66d95bd8c55b378aa2051ac2a0
Signed-off-by: Ronald G. Minnich <rminnich@google.com>
Reviewed-on: http://review.coreboot.org/4091
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

util/cbfstool/elf.h

@@ -243,6 +243,7 @@ typedef struct
 #define EM_ARC_A5	93		/* ARC Cores Tangent-A5 */
 #define EM_XTENSA	94		/* Tensilica Xtensa Architecture */
 #define EM_NUM		95
+#define EM_AARCH64	183		/* ARM AARCH64 */
 
 /* If it is necessary to assign new unofficial EM_* values, please
    pick large random numbers (0x8523, 0xa7f2, etc.) to minimize the