Detect aarch64 and stop emitting AIX-only -maix64 on non-AIX targets

[cvanaret]

How to reproduce the error:

# 1. aarch64 cross-compiler (Debian/Ubuntu)
sudo apt-get install -y gcc-aarch64-linux-gnu g++-aarch64-linux-gnu cmake

# 2. minimal aarch64 cross toolchain file
cat > aarch64.cmake <<'EOF'
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR aarch64)
set(CMAKE_C_COMPILER   aarch64-linux-gnu-gcc)
set(CMAKE_CXX_COMPILER aarch64-linux-gnu-g++)
EOF

# 3. configure + build
cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=aarch64.cmake -DBUILD_SHARED_LIBS=ON
cmake --build build --target asl2

The error reads:

aarch64-linux-gnu-gcc: error: unrecognized command-line option ‘-m64’

See JuliaPackaging/Yggdrasil#13950