Support Apple Silicon and aarch64 Linux

This PR adds support for aarch64 hosts, most commonly Apple Silicon Macs. Updating the version of config.guess from the ancient copy bundled with the deps makes everything work. Surprisingly, it seems like all of the GNU tools built for aarch64 even back in 2006, despite the architecture not having… existed.

Just tested for backwards compat, looks good. Thanks for looking into this.

mentioned in commit d877222c

merged