Let cloudabi-clang install a compiler for aarch64.

Now that CloudABI has been ported over to aarch64, let's extend the
FreeBSD ports to install a functioning toolchain for it.

This change extend the llvm37 port to backport a tiny change that is
needed to make Clang support the CloudABI for aarch64 target (r250416).
This change makes Clang use the right ELFOSABI number, but also makes it
set the right #defines (e.g., CloudABI).

It also extends the cloudabi-clang port to set up symlinks against Clang
for aarch64.

Submitted by: ed
Differential Revision: https://reviews.freebsd.org/D3906