gcc-arm-none-eabi安装后，找不到libgcc.a的拉置
位置在：
/usr/lib/gcc/arm-none-eabi/6.3.1/libgcc.a
查找方法：
arm-none-eabi-gcc --print-libgcc-file-name
以前没找到，是因为进错目录：
/usr/lib/arm-none-eabi/lib
/usr/lib/gcc/arm-none-eabi/6.3.1/libgcc.a
arm-none-eabi-gcc --print-libgcc-file-name
/usr/lib/arm-none-eabi/lib