During the 'link' step, if a package's binary targets don't exist yet
they will fail to be symlinked, even if a later `run xxx` step would
have generated them. This is a situation that occurs in monorepos
containing build tools compiled from another language (e.g. TypeScript).

Simply adding the symlinks whether the target exists or not is not good
enough, as the symlink target needs to be `chmod +x`ed, which is
lerna's/npm's responsibility.

Instead, if the target doesn't exist, generate a shim shell script that
will `chmod` and `exec` the intended target, similar to what would
happen on Windows normally.

Fixes #1444.