Promote riscv64gc-unknown-linux-musl to tier 2

src/ci/docker/host-x86_64/dist-various-2/Dockerfile

@@ -24,7 +24,8 @@ RUN apt-get update && apt-get build-dep -y clang llvm && apt-get install -y --no
   # Needed for apt-key to work:
   dirmngr \
   gpg-agent \
-  g++-9-arm-linux-gnueabi
+  g++-9-arm-linux-gnueabi \
+  gcc-riscv64-linux-gnu
 
 RUN apt-key adv --batch --yes --keyserver keyserver.ubuntu.com --recv-keys 74DA7924C5513486
 RUN add-apt-repository -y 'deb https://apt.dilos.org/dilos dilos2 main'
@@ -74,6 +75,11 @@ RUN env \
     CXX=arm-linux-gnueabi-g++-9 CXXFLAGS="-march=armv7-a" \
     bash musl.sh armv7 && \
     rm -rf /build/*
+RUN env \
+    CC=riscv64-linux-gnu-gcc \
+    CXX=riscv64-linux-gnu-gcc \
+    bash musl.sh riscv64gc && \
+    rm -rf /build/*
 
 WORKDIR /tmp
 COPY scripts/shared.sh /tmp/
@@ -127,6 +133,7 @@ ENV TARGETS=$TARGETS,loongarch64-unknown-none-softfloat
 ENV TARGETS=$TARGETS,aarch64-unknown-uefi
 ENV TARGETS=$TARGETS,i686-unknown-uefi
 ENV TARGETS=$TARGETS,x86_64-unknown-uefi
+ENV TARGETS=$TARGETS,riscv64gc-unknown-linux-musl
 
 # As per https://bugs.launchpad.net/ubuntu/+source/gcc-defaults/+bug/1300211
 # we need asm in the search path for gcc-9 (for gnux32) but not in the search path of the
@@ -137,6 +144,7 @@ RUN ln -s /usr/include/x86_64-linux-gnu/asm /usr/local/include/asm
 ENV RUST_CONFIGURE_ARGS --enable-extended --enable-lld --disable-docs \
   --set target.wasm32-wasi.wasi-root=/wasm32-wasi \
   --set target.wasm32-wasi-preview1-threads.wasi-root=/wasm32-wasi-preview1-threads \
-  --musl-root-armv7=/musl-armv7
+  --musl-root-armv7=/musl-armv7 \
+  --musl-root-riscv64gc=/musl-riscv64gc
 
 ENV SCRIPT python3 ../x.py dist --host='' --target $TARGETS

src/tools/build-manifest/src/main.rs

@@ -132,6 +132,7 @@ static TARGETS: &[&str] = &[
     "riscv64gc-unknown-hermit",
     "riscv64gc-unknown-none-elf",
     "riscv64gc-unknown-linux-gnu",
+    "riscv64gc-unknown-linux-musl",
     "s390x-unknown-linux-gnu",
     "sparc64-unknown-linux-gnu",
     "sparcv9-sun-solaris",