Cannot run foreign architecture AppImages directly with binfmt_misc support

I cannot directly run foreign architecture (ARM, ARM64) AppImages on an x64 installation even with binfmt-support and qemu-user-static installed.

Trying to run `appimagetool-armhf.AppImage` gives:

```
~/$ ./appimagetool-armhf.AppImage
bash: ./appimagetool-armhf.AppImage: Cannot execute binary file: Exec format error
```

I can run it by explicitly invoking qemu-arm-static though:

```
~/$ qemu-arm-static ./appimagetool-armhf.AppImage
appimagetool, continuous build (commit effcebc), build 2084 built on 2019-05-01 21:02:27 UTC
SOURCE is missing
```

I can run other ARM executables directly without explicitly invoking qemu:

```
~/$ arm-linux-gnueabihf-gcc helloworld.cpp -o helloworld
~/$ file helloworld
helloworld: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-, for GNU/Linux 3.2.0, BuildID[sha1]=1b96535ea748e1c22960251f84ea72c0ff1a45df, not stripped
~/$ ./helloworld
Hello world!
```
Would be great if this could be made to work for AppImages too.