SiFive.com seem to be closest to having 64 bit hardware. They came out with 32 bit hardware a few weeks back (which doesn't run Linux -- it's a sort of Arduino type board). LowRisc.org may have 64 bit hardware next year. There are a few others, best to see the recent conference talks for details: https://riscv.org/2016/12/5th-risc-v-workshop-proceedings/
