Initial arm64 / aarch64 support

Initial arm64 / aarch64 support

Relevant changes:

* arm64 (ARM64 in terms of FAI class) is considered a valid and
  supported architecture within grml-live now; updated documentation,
  configuration files and zsh completion accordingly

* arm64 was enabled as supported architecture in grml repositories

* the architecture specific grml-scripts + grml-scripts-core packages
  got built for arm64 and uploaded to the grml-testing repository

* memtest86+ + syslinux aren't available on arm64, therefore move
  memtest86+ to Recommends to avoid ending up with an `Arch: any`
  package, also depend on either syslinux or syslinux-efi

* Software related changes in GRML* (other than the obvious
  linux-image-arm64 and further arm64 specific packages):

  * GRMLBASE:

    - grml2usb is I386 + AMD64 specific for now (if we want to use
      grml2usb on arm64 as well, we'd need to port it)

    - grub-efi-amd64-bin, grub-efi-ia32-bin + grub-pc as well as
      syslinux, syslinux-common + syslinux-utils all are also
      I386 + AMD64 specific

  * GRML_FULL:

    - cciss-vol-status, cmospwd, cpuid, grub-pc-bin + ltrace all
      are I386 + AMD64 specific (so not available for arm64)

    - grml-terminalserver is I386 + AMD64 specific for now (at least
      until we ported grml2usb for arm64)

* /etc/grml/fai/config/scripts/GRMLBASE/44-grub: invocation of
  grub-mkimage is I386 + AMD64 specific (at least until we identify
  whether there's an actual use-case for our generated
  /boot/grub/core.img)

* /etc/grml/fai/config/scripts/GRMLBASE/45-grub-images: we use
  /boot/bootaa64.efi for the EFI boot image name for arm64, though need
  to skip the unavailable efi_uga module and also skipt the i386-pc
  architecture, since grub on arm64 doesn't provide /usr/lib/grub/i386-efi

Known TODOs:

* decide official naming for Grml on arm64 (also keep e.g.
  `update-grml-rescueboot -t small -a 64` in mind!)

* grml2usb isn't available for arm64 yet, and therefore also
  grml-terminalserver isn't available for arm64

* when booting the resulting ISO inside an arm64 / aarch64, the default
  terminal seems to start up as /dev/ttyAMA0 (instead of /dev/tty1, as
  expected and handled via our getty@tty1.service)

* look into SecureBoot support

Development sponsored by netcup GmbH