[PATCH] Add target hook in C front end

[Eric Christopher]

> Or, possibly even better, get the driver to pass the correct -D
> commands on the command line.  This doesn't require the front ends to
> be integrated with all preprocessors, and is what would probably
> happen if I ever do my specs replacement.

This is what the mips backend does currently, just via specs files. If
you want to see just how bad this is in general take a look at
gcc/config/mips/isa3264.h. That was a first attempt at doing this in a
general way - and doesn't work. As I'm trying to donate additional mips
targets I run into many instances of this, all different and all not
sufficient. :)

A specs replacement where I could use C to define what is passed would
be wonderful :)

-eric

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