build-llvm: Add an option --no-llvm-tool-reuse

This allows using the build-llvm.sh script for doing a cross build
of LLVM, while the same directory may contain a native build
which can be out of sync with the actual source.

build-cross-tools.sh

@@ -31,6 +31,9 @@ while [ $# -gt 0 ]; do
     --disable-clang-tools-extra)
         LLVM_ARGS="$LLVM_ARGS $1"
         ;;
+    --no-llvm-tool-reuse)
+        LLVM_ARGS="$LLVM_ARGS $1"
+        ;;
     --disable-mingw-w64-tools)
         NO_MINGW_W64_TOOLS=1
         ;;