Also install [...]/nvptx-none/bin/ar and [...]/nvptx-none/bin/ranlib.

GCC needs this, if nvptx-none-ar and nvptx-none-ranlib aren't found in $PATH.
Support standard compiler configuration variables.

Also remove the LIBPATH define, which is not used anywhere.
Don't link the assembler and linker against CUDA libraries.

..., because that's not needed.