Remaining host configuration fragments

[DJ Delorie]

> It generally goes "works" --> bitrotten (for several releases) -->
> "missing" in practice.  People can always volunteer to add back and
> maintain support for a desupported host or target, getting the last
> version of support files from CVS.

Bitrotting is a poor way to officially obsolete a host or target.  I
would suggest modifying config.gcc to support a two-step obsolescence:

1. Deprecate.

$ ./configure
Warning: mips-foo-bar is deprecated and will be removed in a future
release.
Use --enable-deprecated-configs to build it anyway.
... stops ...

$ ./configure --enable-deprecated-configs
Warning: mips-foo-bar is deprecated and will be removed in a future
release.
... continues ...

2. Obsolete.

$ ./configure
Error: target mips-foo-bar is obsolete.  If you wish to re-add support
for this target, you may volunteer to do so.
... stops ...