Get rid of common syms (#2040)

src/fieldmap.c

@@ -31,7 +31,7 @@ EWRAM_DATA struct Camera gCamera = {0};
 EWRAM_DATA static struct ConnectionFlags sMapConnectionFlags = {0};
 EWRAM_DATA static u32 UNUSED sFiller = 0; // without this, the next file won't align properly
 
-struct BackupMapLayout gBackupMapLayout;
+COMMON_DATA struct BackupMapLayout gBackupMapLayout = {0};
 
 static const struct ConnectionFlags sDummyConnectionFlags = {0};