Fix quoting of imported object files in calls to spawn

This allows me to compile all of Platform in one go (there was no issue with command line length).

Linker errors are sent to stderr by the linker, so they were lost by clm. I added code to redirect them to linkerrs.$$$ (following the naming convention of cgopt.$$$/linker.$$$), and subsequently print the errors and remove the file again. I know that you can do this in memory with a separate thread (as is done with the compiler), but I couldn't get this to work so quickly, we don't need live redirection, and it doesn't really seem worth the extra code to clean up the threads etc. afterwards. So I hope this is OK.