Adding aarch64.apple.darwin9 or arm64.apple.darwin9 support for M1 MacBooks

[bpecsek]

Could you please add support for the M1 MacOSX?

After installing clang-11 from homebrew c2ffi can now be built on the M1 Macbook using this shell script. ''' git clone https://github.com/rpav/c2ffi.git cd c2ffi mkdir build cd build/ LLVM_DIR=/opt/homebrew/Cellar/llvm/11.1.0_1/lib/cmake/llvm CC="clang-11" CXX="clang++" cmake .. LIBRARY_PATH=/opt/homebrew/Cellar/llvm/11.1.0_1/Toolchains/LLVM11.1.0.xctoolchain/usr/lib make '''