Add arm64 to universe if binutils is available.

arm64 relies on an external binutils port or package right now, because
the in-tree linker from binutils 2.17.50 does not support arm64. Add
arm64 to universe if the linker is available. If not output a message
that arm64 is skipped.

buildworld and buildkernel use the external binutils automatically, so
it's sufficient to run 'pkg install aarch64-binutils' to build
FreeBSD/arm64.

Differential Revision: https://reviews.freebsd.org/D2302
Reviewed by: andrew, imp
Sponsored by: The FreeBSD Foundation