Work has moved into the FSF GDB source tree. See http://sourceware.org/gdb This tree is now obsolete, except for the info about linux kernel and glibc patches.
At the time this was written, my branch requires 4 Linux Kernel patches. A patch file that includes all four of these patches is in riscv-linux.txt in the top level dir. The first patch has been added into the official tree. It can be found at https://patchwork.kernel.org/patch/10458899 The second patch is in the queue to be applied soon. It can be found at https://patchwork.kernel.org/patch/10524925/ The third patch is in the queue to be applied soon. It can be found at riscvarchive/riscv-linux#140 and involves fixing arch/riscv/include/uapi/asm/syscalls.h to remove the include guards. The fourth patch is an initial attempt to add FP register ptrace support. This patch requires more work, on both the linux kernel and gdb sides.
There is also a GLIBC patch required for thread debugging support. https://sourceware.org/ml/libc-alpha/2018-07/msg00303.html