Adding FreeBSD/arm64 Jenkins build / QEMU job

[Ed Maste]

As of Andrew's bug fixes earlier today (r281556 et al) we can build
FreeBSD/arm64 in SVN HEAD (targeting QEMU). Because the in-tree GNU ld
2.17.50 does not support AArch64 we rely on having the
aarch64-binutils port or package installed. Other than that arm64
builds with the standard in-tree Clang/LLVM tool chain.

As an example I've started a local build using the Jenkins build1.sh
script from https://github.com/freebsd/freebsd-ci with:

% WORKSPACE=/tank/scratch MAKE_CONF_FILE="TARGET=arm64" sh
~/src/freebsd-ci/scripts/build/build1.sh

Can we add this as a Jenkins job? We can later extend it to build and
boot an image in QEMU and run Kyua tests but just having the build job
is a nice start.