Add riscv64gc to CI

There is a proposal to promote `riscv64gc-unknown-linux-gnu` to tier 1
[1]. We do not currently test RISC-V in CI; add it here.

[1]: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Imminent.20RFC.20PR.3A.20riscv64gc-unknown-linux-gnu.20to.20Tier-1

.github/workflows/main.yml

@@ -61,6 +61,9 @@ jobs:
         - target: powerpc64le-unknown-linux-gnu
           os: ubuntu-latest
           rust: nightly
+        - target: riscv64gc-unknown-linux-gnu
+          os: ubuntu-latest
+          rust: nightly
         - target: thumbv6m-none-eabi
           os: ubuntu-latest
           rust: nightly

ci/docker/riscv64gc-unknown-linux-gnu/Dockerfile

@@ -0,0 +1,13 @@
+ARG IMAGE=ubuntu:24.04
+FROM $IMAGE
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    gcc libc6-dev qemu-user-static ca-certificates \
+    gcc-riscv64-linux-gnu libc6-dev-riscv64-cross \
+    qemu-system-riscv64
+
+ENV CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_LINKER=riscv64-linux-gnu-gcc \
+    CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_RUNNER=qemu-riscv64-static \
+    QEMU_LD_PREFIX=/usr/riscv64-linux-gnu \
+    RUST_TEST_THREADS=1