clang_llvm toolchain installation

#Install clang/llvm

$ git clone https://github.com/plctlab/llvm-project.git
$ cd llvm-project
$ mkdir build
$ cd build
$ sudo apt install build-essential libssl-dev
$ wget https://github.com/Kitware/CMake/releases/download/v3.20.2/cmake-3.20.2.tar.gz
$ tar -zxvf cmake-3.20.2.tar.gz
$ cd cmake-3.20.2
$ ./bootstrap
$ make 
$ sudo make install 
$ cmake -DLLVM_TARGETS_TO_BUILD="RISCV" -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_USE_LINKER=gold -G Ninja ../llvm
$ ninja
$ sudo cmake --build . --target install

#run test

$ /home/qiao_sh_pudong/RISCV/Src/llvm-project/build/bin/clang++  --target=riscv64-unknown-elf -march=rv64gcv1p0 -menable-experimental-extensions --sysroot=/home/qiao_sh_pudong/RISCV/Bin/riscv64/riscv64-unknown-elf --gcc-toolchain=/home/qiao_sh_pudong/RISCV/Bin/riscv64 hello.cpp
$ spike --isa=rv64gcv pk a.out