Remaining host configuration fragments

[Joe Buck]

Kenner writes:

> I'd be very hesitant to remove a back-end that was recently added to GCC
> no matter how little use we think it's getting.

In this case [pdp11] it seems that several maintainers are willing to
spend a few cycles hacking on it, so let's just leave it.

On the other hand, the elxsi backend is still there.  Mike Stump,
didn't you let us know a few years ago that the last ELXSI machine
had been turned off some time ago?