gcc-wrapper: add 'cpp' wrapper

The gcc-wrapper doesn't wrap 'cpp'. This breaks some software (such as Buildroot) because the 'cpp' they get come from the non-wrapped gcc package which doesn't know about any standard include paths.

I also added wrapping for cross-'cpp', but I didn't test it. It's "identical" to the native 'cpp' wrapping, so I expect it to work.

WARNING: massive rebuilds.