cc-wrapper, bintools-wrapper: simply symlink man and info outputs

With the previous commit `propagateDoc` is now always given the correct value
(i.e. it is never set to `true` when there are no `man` and `info` outputs).
Hence, we can simply symlink the original outputs to the wrapper outputs.

Pros:

- simpler, less indirection compared to `propagated-user-env-packages`,
- uses less inodes (1 symlink, which nix then simply automatically resolves
  and removes, vs. two directories and a file),
- makes direct references like "export MANPATH=${stdenv.cc.man}/share/man"
  simply work.

Cons:

- I'm not aware of any.

This and the previous commit together almost completely revert commits
fde7296a,
fa412972, and
c981787d.