Start to import support for the AArch64 architecture from ARM. This change
only adds support for kernel-toolchain, however it is expected further
changes to add kernel and userland support will be committed as they are
reviewed.

As our copy of binutils is too old the devel/aarch64-binutils port needs
to be installed to pull in a linker.

To build either TARGET needs to be set to arm64, or TARGET_ARCH set to
aarch64. The latter is set so uname -p will return aarch64 as existing
third party software expects this.

Differential Revision: https://reviews.freebsd.org/D2005
Relnotes: Yes
Sponsored by: The FreeBSD Foundation