Yufeng Zhang has recently posted that support for the aarch64 architecture (ie., 64-bit ARMv8 processors) has now landed in the official binutils git repository. This code should make its way into the next release of binutils, version 2.23. The relevant commit is e7dff39c, the ChangeLog update is here.

I've updated the ARM64 toolchain document to refer to the upstream sources, and a test build runs without problems. Nice work, ARM team!