x86emul: extend / amend supported FPU opcodes

x86emul: extend / amend supported FPU opcodes

First of all there are a number of secondary encodings both Intel and
AMD support, but which aren't formally documented. See e.g.
www.sandpile.org/x86/opc_fpu.htm for inofficial documentation.

Next there are a few more no-ops - instructions which served a purpose
only on 8087 or 287.

Further switch from fail_if() to raising of #UD in a couple of places
(as the decoding of FPU opcodes should now be complete except where
explicitly marked as todo).

Also adjust a few comments.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>