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.
- there were many easy merge conflicts
- cc-wrapper needed nontrivial changes
Many other problems might've been created by interaction of the branches,
but stdenv and a few other packages build fine now.
Problems are worked around by using older gcc wrapper for gnat bootstrap.
(from ca441636f1^)
I tried nicer solutions first, but I was unable to fix all problems for hours.
