arm64 diagnostic when running 'make tinderbox'

[Kurt Lidl]

Greetings all -

I periodically run the following on one of my hosts
(an amd64 machine, running 10.3+patches), to make sure
that I haven't broken the build for clang and gcc 4.2.1
based architectures before committing.

I've noticed that I *always* get a diagnostic message
about arm64, even though I'm not compiling for it...

   root at hydra-892: make -k -s -j 24 tinderbox TARGETS="sparc64 amd64"
   --------------------------------------------------------------
   >>> Building an up-to-date bmake(1)
   --------------------------------------------------------------
   --------------------------------------------------------------
   >>> make universe started on Fri Jun 10 09:40:53 EDT 2016
   --------------------------------------------------------------
   >> arm64 skipped - install aarch64-binutils port or package to build
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   >> amd64 started on Fri Jun 10 09:40:53 EDT 2016
   >> sparc64 started on Fri Jun 10 09:40:53 EDT 2016
   >> amd64.amd64 buildworld started on Fri Jun 10 09:40:53 EDT 2016
   >> sparc64.sparc64 buildworld started on Fri Jun 10 09:40:53 EDT 2016

-Kurt