Remaining host configuration fragments

[DJ Delorie]

> In contrast with some other folks, I would even be willing to say
> something like this: "Even though we know there are people out there
> who still use Irix 5, we are dropping support for it in GCC 3.1.
> You can still use GCC 3.0 (or earlier) there, of course."

Like all other gcc features, we need at least one release where it
exists but is deprecated, perhaps with a warning printed for each gcc
invocation.  Support for a host shouldn't go from "works" to "missing"
in one step - then someone willing to support it has no chance of
volunteering until it's too late.