x86/Intel: correct disassembly of fsub*/fdiv*

fsub/fsubr/fsubp/fsubrp as well as fdiv/fdivr/fdivp/fdivrp disassembly
should match (a) the Intel SDM and (b) respective input fed to gas (both
of course with the exception of when we intentionally convert bogus
insns, accompanied by a warning).