IR generation: speed up by creating empty files

[arichardson]

If the binaries created by the project aren't required for code generation it would be a lot faster to simply create empty files for the .o, .a and .so files so that makefile dependencies work, but no real compiler invocation is needed.
Should be configurable using e.g. an env var so that required targets can still be built (maybe by filtering on the directory) but all others are skipped.