Install riscv linux musl toolchain in travis ci

.travis.yml

@@ -81,6 +81,25 @@ install:
     travis_wait brew install x64-elf-gcc;
     fi;
     fi
+  - if [ $ARCH = riscv32 ]; then
+    if [ $TRAVIS_OS_NAME = linux ]; then
+    sudo apt update;
+    sudo apt install linux-headers-$(uname -r);
+    wget https://musl.cc/riscv32-linux-musl-cross.tgz;
+    tar -xvf riscv32-linux-musl-cross.tgz;
+    export PATH=$PATH:$PWD/riscv32-linux-musl-cross/bin;
+    fi;
+    fi
+  - if [ $ARCH = riscv64 ]; then
+    if [ $TRAVIS_OS_NAME = linux ]; then
+    sudo apt update;
+    sudo apt install linux-headers-$(uname -r);
+    wget https://musl.cc/riscv64-linux-musl-cross.tgz;
+    tar -xvf riscv64-linux-musl-cross.tgz;
+    export PATH=$PATH:$PWD/riscv64-linux-musl-cross/bin;
+    fi;
+    fi
+
 
 before_script:
   - rustup component add rust-src