The 64-bit ARM (AArch64) port image of Debian/Ubuntu has surfaced. Debian-based Linux is now ready to play in a 64-bit ARM world, months ahead of any hardware appearing for the general public. Similar to x86_64, Linux is the first operating system ready for the new architecture.
For months the upstream Linux work has been happening by ARM Holdings with GCC compiler support, LLVM/Clang compiler support, Linux kernel support, and Glibc, among other components. This work has now all been merged into the respective mainline code-bases so it's really up to the distributors now to have a hand at making their software 64-bit ARM ready.