functions.sh: add aarch64_be target

aarch64 in big-endian mode is an interesting architecture choice because it's a big-endian environment that can work on a lot of widely available hardware (unlike s390x) which isn't horribly outdated in performance and memory size (unlike 32-bit PowerPC).

I've run aport's bootstrap.sh script with this and a few other patches. The resulting binaries work ok.

Open questions:

• should the name be aarch64be or aarch64_be or even aarch64eb?
  • -> aarch64_be