[cmucl-imp] Removing x87 support

[Raymond Toy]

SSE2 has been supported by CMUCL since around 2008 and SSE2 was introduced
in the Pentium 4 in 2001. It's now time to drop support for x87.

I suspect x87 is rarely, if ever, used anymore since CMUCL will
automatically detect SSE2 and use that if your CPU supports. I rarely test
x87 except to see that it starts when a snapshot is made. Removing this
support will make the tarballs half as large (no x87 core anymore), a large
chunk of code can be removed as well as simplifying the hairy x86-assem.S
file.

If you need x87 support speak up!  No guarantees that we'll keep x87
support, but knowing that it's important to you will help.

The current plan is that the upcoming April snapshot will be the last
version that will ship with x87 support. After that, tarballs will not ship
with the x87 core file and we will begin removing support for x87.

Ray