dtoolbase: add missing DTOOL_PLATFORM for linux_aarch64

dtool/src/dtoolbase/dtool_platform.h

@@ -63,6 +63,9 @@
 #define DTOOL_PLATFORM "android_i386"
 #endif
 
+#elif defined(__aarch64__)
+#define DTOOL_PLATFORM "linux_aarch64"
+
 #elif defined(__x86_64)
 #define DTOOL_PLATFORM "linux_amd64"