Let cloudabi-clang install a compiler for aarch64.

[ed]

Summary

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.

Test Plan

This change has been in use locally for some time and it seems to work.

[ed]

No need to call mkdir twice.

[emaste]

seems sensible to me

[brooks]

I'll commit this (slightly modified)