Hard link input_files in a tmp directory to save space

While preparing input_files for building a project, we copy them in a temporary directory. When we build a lot of projects recursively, this can take a lot of space (especially for the Android builds where the toolchain takes 1.6G).

To avoid this I think we can make hard links instead of copying the files, at least in the case where remote_exec is used (as we don't do the build from this temporary directory, but copy all files using remote_put in this case), and as an option when remote_exec is not used.