Update servodemo build instructions to consider build mode support for prebuilt libraries #4
Description
cpp构建的缓存目录增加一层当前buildMode的文件夹，目录结构由.cxx/default/default/arm64-v8a改为.cxx/default/default/debug/arm64-v8a或.cxx/default/default/release/arm64-v8a。
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