Fix FSR3 permutation script failures on arm64 Linux and Windows

Linux arm64 (Exec format error):
- The script was downloading the x86_64 DXC release on all Linux hosts;
  on aarch64 runners the x86_64 ELF fails with EXEC_FORMAT_ERROR at
  shader compilation time. Add uname -m guard: when running on aarch64/
  arm64 Linux without a DXC in PATH, exit 0 with an advisory message
  rather than downloading an incompatible binary. The FSR3 SDK build
  proceeds as it did before the permutation script was introduced
  (permutation headers are expected to be pre-built in the SDK checkout).

Windows (bash: command not found, exit 127):
- cmake custom-target COMMANDs run via cmd.exe on Windows even when
  cmake is configured from a MSYS2 shell, so bare 'bash' is not resolved.
- Use find_program(BASH_EXECUTABLE bash) at configure time (which runs
  under shell: msys2 in CI and thus finds the MSYS2 bash at its native
  Windows-absolute path). When bash is found, embed the full path in the
  COMMAND; when not found (unusual non-MSYS2 Windows setups), skip the
  permutation step and emit a STATUS message.

tools/generate_ffx_sdk_vk_permutations.sh

@@ -35,6 +35,13 @@ if [[ -z "${DXC:-}" ]]; then
   elif command -v dxc >/dev/null 2>&1; then
     export DXC="$(command -v dxc)"
   elif [[ "$(uname -s)" == "Linux" ]]; then
+    _arch="$(uname -m)"
+    if [[ "$_arch" == "aarch64" || "$_arch" == "arm64" ]]; then
+      echo "Linux aarch64: no official arm64 DXC release available." >&2
+      echo "Install 'directx-shader-compiler' via apt or set DXC=/path/to/dxc to regenerate." >&2
+      echo "Skipping VK permutation codegen (permutations may be pre-built in the SDK checkout)."
+      exit 0
+    fi
     echo "DXC not found; downloading Linux DXC release to /tmp/dxc ..."
     tmp_json="$(mktemp)"
     curl -sS https://api.github.com/repos/microsoft/DirectXShaderCompiler/releases/latest > "$tmp_json"