Arm32_64_USB Pi3-64 packing problems


Using a compiler newer than 7.4.1 seems to cause __pack compilation problems when trying to `make Pi3-64` in the   Arm32_64_USB directory.
```
$aarch64-linux-gnu-gcc --version
aarch64-linux-gnu-gcc (Debian 8.3.0-2) 8.3.0
```
