native build on arm64 failed

An error occurs when building the image on a machine with a native arm64 architecture: ``` cargo:warning=Compiler family detection failed due to error: ToolExecError: command `"/root/aarch64-linux-musl-cross/bin/aarch64-linux-musl-gcc" "-E" "/usr/local/src/shadowsocks-rust/target/aarch64-unknown-linux-musl/release/build/ring-d14434f0281c11dc/out/17838610413816276646detect_compiler_family.c"` failed to start: Exec format error (os error 8) ``` On other architectures, you should use the native GCC compiler instead of a cross-toolchain build.