mips64 ieee754 data structures

[Roland McGrath]

> Thius is stupid.  We have separate files for exactly the purposes of
> keeping unneeded and wrong definitions away.

We have separate files for the purpose of keeping things cleanly separate
when they are in fact different.

Right now the source tree has 5 ieee754.h files that are all almost identical.
That is just stupid.  (In fact, two are completely identical.)

When I wrote this file originally, it was an internal header only; so then
duplication could have been avoided using #include.  Now that it's an
installed header (I don't know why it become one), that's not possible.  
If we can replace 5 copies in 5 files with 2 copies in one file, I call that
an improvement.  It also leaves less for a new port to do, and less to be done
for a new configuration on an existing machine that changes the `long double'
size from what was used before.


Thanks,
Roland