RFD: hookizing BITS_PER_UNIT in tree optimizers / frontends

[Joern Rennecke]

Quoting Richard Guenther <richard.guenther@gmail.com>:

> Well.  Some things really ought to stay as macros.  You can always
> error out if a multi-target compiler would have conflicts there at
> configure time.

So what are we going to do about all the tree optimizers and frontends that
use BITS_PER_UNIT?
Should they all include tm.h, with the hazard that more specific
macros creep in?
Or do we want to put this in a separate header file?