x86: Disable generation of traditional x87 instructions

We recently had the case where wrongly used floating-constant 'E' caused
the generation of traditional x87 instructions in kernel code and
wreaking all kinds of havoc.

Disable the generation of those too. This will save people a lot of time
when trying to debug such issues by erroring out of the build instead of
let them manifest themselves in very spectacular and happy-crappy ways
at runtime.

We're using -mno-fp-ret-in-387 in addition to -mno-80387 (which is ==
-msoft-float) because, as the gcc manpage says:

  On machines where a function returns floating-point results in the
  80387 register stack, some floating-point opcodes may be emitted even
  if -msoft-float is used.

so we want to turn off *all* non-integer instructions involving any
architectural FPU state, unless it is absolutely necessary (and those
cases need special handling anyway).

Cc: Jiri Kosina <[email protected]>
Cc: Michael Matz <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: H. Peter Anvin <[email protected]>

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