an optional choice for the toolchain

If the submodule AME_llvm in this repository is unavailable, please

1. Download this file to the directory

$GEM5_HOME/toolchain/AME_llvm

2. Then, run the toolchain build script:

   bash $GEM5_HOME/scripts/build_toolchain.sh

Note: Prebuilt binaries require an x86_64 Linux host with glibc >= 2.31 and libstdc++ providing GLIBCXX_3.4.26 or newer.