RFC: semi-automatic hookization

[Joern Rennecke]

Quoting Paolo Bonzini <bonzini@gnu.org>:

> Augmenting libcpp with a new kind of poisoning that only affect
> preprocessor conditionals would probably make this a lot simpler.

If we don't have a wrapper macro, we can just poison the macro.
I still have to find out how practical that is, but for now let's assume
we can do that.

How about this:
- make a new header tm-poison.h that poisons all target macros, and sets the
   GCC_TM_H include guard.
- include this header in system.h for IN_GCC_FRONTEND
- include this header in tree.h / gimple.h unless GCC_TM_H has been set