Add aarch64 as a configuration

[henriksod]

Once this issue #144 is resolved, I suggest we add aarch64 as a configurable target architecture so any future changes will also compile towards aarch64.

I have already added this in my own project using the gcc_toolchain repo.

I guess for clang we could use https://github.com/grailbio/bazel-toolchain to configure for aarch64?

[mvukov]

Sounds good. FWIW, it looks to me that grailbio/bazel-toolchain is more actively developed than gcc_toolchain.