如何構建RISC-V的交叉編譯工具鏈？

1. 創建risc-v目錄
   $ mkdir risc-v
   $ cd risc-v
2. 獲取源碼
   $ git clone --recursive https://github.com/riscv/riscv-gnu-toolchain.git
3. 安裝依賴包
   $ sudo apt-get install autoconf automake autotools-dev curl libmpc-dev libmpfr-dev
   libgmp-dev gawk build-essential bison flex texinfo gperf libtool patchutils bc zlib1g-dev libexpat-dev (on ubuntu 18.04)
4. 編譯並安裝
   $ cd riscv-gnu-toolchain
   $ ./configure --prefix=/opt/riscv
   $ make (newlib cross-compiler, a generic ELF toolchain, select generic ELF toolchain)
   $ sudo make install
5. 配置環境變量
   $ export RISCV=/opt/riscv
   $ export PATH=/opt/riscv/bin:$PATH