Please add aarch64 support for misc/raspberrypi-userland

[ykla]

Now raspberrypi-userland supports aarch64 on Linux.
https://github.com/raspberrypi/userland
says:
*Whilst 64-bit userspace is not officially supported, some of the libraries
will work for it. To cross compile, install gcc-aarch64-linux-gnu and
g++-aarch64-linux-gnu first. For both native and cross compiles, add the
option --aarch64 to the buildme command.  *
And I test in Debian and Gentoo, all works. Maybe also works well in
FreeBSD. So please update it and adds arm64 support.