(this is a copy of the message I sent on #stdenv:nixos.org)
stdenv is currently broken for riscv64-linux in staging due to the 5-year-old assembler in the bootstrap tarball not supporting the vector extension. I built a new one (freshBootstrapTools.build on 5a09cb4) and verified that it fixes the issue.
@lovesegfault I was told by @RaitoBezarius to ping you for upload on tarballs.nixos.org
Related: #253713