cc-wrapper, cc-wrapper-old: Simplify shell logic

1. `crossDrv` is now the default so we don't need to worry about that in
   build != host builds.

2. shell is the build time shell, so `wrapCCCross` doesn't need to
   worry, as build == host.

3. `shell.shellPath` will always be appended where useful.

4. Complicated `shell == ""` logic served no purpose.