add a riscv64 test job

Author: RalfJung

.github/workflows/ci.yml

@@ -36,6 +36,11 @@ jobs:
             multiarch: s390x
             gcc_cross: s390x-linux-gnu
             qemu: true
+          - host_target: riscv64gc-unknown-linux-gnu
+            os: ubuntu-latest
+            multiarch: riscv64
+            gcc_cross: riscv64-linux-gnu
+            qemu: true
           - host_target: aarch64-apple-darwin
             os: macos-latest
           - host_target: i686-pc-windows-msvc