Introduce Binutils for CloudABI on aarch64.

When I added the first copy of the CloudABI toolchain to the Ports tree,
I assumed that it would be easily possible to have a single Binutils
port that would support all of the architectures of interest. It seems
that this is not really supported, or simply awkward to use.

Let's just rename the cloudabi-binutils port to cloudabi-binutils-x86_64
and add an additional cloudabi-binutils-aarch64.

Reviewed by: emaste
Approved by: bapt
Differential Revision: https://reviews.freebsd.org/D3919