x86/math-emu: Fix function cast warnings

clang-16 warns about casting function pointers with incompatible
prototypes. The x86 math-emu code does this in a number of places
to call some trivial functions that need no arguments:

  arch/x86/math-emu/fpu_etc.c:124:14: error: cast from 'void (*)(void)' to 'FUNC_ST0' \
    (aka 'void (*)(struct fpu__reg *, unsigned char)') converts to incompatible function \
    type [-Werror,-Wcast-function-type-strict]
    124 |         fchs, fabs, (FUNC_ST0) FPU_illegal, (FUNC_ST0) FPU_illegal,
        |                     ^~~~~~~~~~~~~~~~~~~~~~

  arch/x86/math-emu/fpu_trig.c:1634:19: error: cast from 'void (*)(void)' to 'FUNC_ST0' \
    (aka 'void (*)(struct fpu__reg *, unsigned char)') converts to incompatible function \
    type [-Werror,-Wcast-function-type-strict]
   1634 |         fxtract, fprem1, (FUNC_ST0) fdecstp, (FUNC_ST0) fincstp
        |                          ^~~~~~~~~~~~~~~~~~

  arch/x86/math-emu/reg_constant.c:112:53: error: cast from 'void (*)(void)' to 'FUNC_RC' \
  (aka 'void (*)(int)') converts to incompatible function \
  type [-Werror,-Wcast-function-type-strict]
    112 |         fld1, fldl2t, fldl2e, fldpi, fldlg2, fldln2, fldz, (FUNC_RC) FPU_illegal

Change the fdecstp() and fincstp() functions to actually have the correct
prototypes based on the caller, and add wrappers around FPU_illegal() for
adapting those.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/lkml/20240213095631.454543-1-arnd@kernel.org