x86/math-emu: Limit MATH_EMULATION to 486SX compatibles

The FPU emulation code is old and fragile in places, try to limit its
use to builds for CPUs that actually use it. As far as I can tell,
this is only true for i486sx compatibles, including the Cyrix 486SLC,
AMD Am486SX and ÉLAN SC410, UMC U5S amd DM&P VortexSX86, all of which
were relatively short-lived and got replaced with i486DX compatible
processors soon after introduction, though some of the embedded versions
remained available much longer.

Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Bill Metzenthen <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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