Hi.
After the FSF Ada compler GNAT was not available on Archlnux ARM you can now install it with an Alire (Ada package manager) repro. This is not an Archlinux package, but for once you at leasrt can get Ada on Archlinux ARM.
[url]https://github.com/reznikmm/aarch64-alire-index[/url]
Install latest alire package and add to the path, then follow the README to install the rest of the Ada toolchain.