Use lib instead of CMAKE_INSTALL_LIBDIR for pioasm install

[will-v-pi]

The CMake GNUInstallDirs module prior to CMake version 3.23.1 used an architecture-specific CMAKE_INSTALL_LIBDIR on debian, which is not searched by find_package calls from the SDK

Fix this in the same way as picotool https://github.com/raspberrypi/picotool/issues/117 by hard-coding the lib directory

[lurch]

Fix this in the same way as picotool raspberrypi/picotool#117 by hard-coding the lib directory

For cross-reference porpoises :dolphin: https://github.com/raspberrypi/picotool/commit/6ad9c2352cf90b4abf183143e3cbd9ec933b9ad1