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

arch/x86/math-emu/fpu_etc.c

@@ -120,9 +120,14 @@ static void fxam(FPU_REG *st0_ptr, u_char st0tag)
 	setcc(c);
 }
 
+static void FPU_ST0_illegal(FPU_REG *st0_ptr, u_char st0_tag)
+{
+	FPU_illegal();
+}
+
 static FUNC_ST0 const fp_etc_table[] = {
-	fchs, fabs, (FUNC_ST0) FPU_illegal, (FUNC_ST0) FPU_illegal,
-	ftst_, fxam, (FUNC_ST0) FPU_illegal, (FUNC_ST0) FPU_illegal
+	fchs, fabs, FPU_ST0_illegal, FPU_ST0_illegal,
+	ftst_, fxam, FPU_ST0_illegal, FPU_ST0_illegal,
 };
 
 void FPU_etc(void)

arch/x86/math-emu/fpu_trig.c

@@ -433,13 +433,13 @@ static void fxtract(FPU_REG *st0_ptr, u_char st0_tag)
 #endif /* PARANOID */
 }
 
-static void fdecstp(void)
+static void fdecstp(FPU_REG *st0_ptr, u_char st0_tag)
 {
 	clear_C1();
 	top--;
 }
 
-static void fincstp(void)
+static void fincstp(FPU_REG *st0_ptr, u_char st0_tag)
 {
 	clear_C1();
 	top++;
@@ -1631,7 +1631,7 @@ static void fscale(FPU_REG *st0_ptr, u_char st0_tag)
 
 static FUNC_ST0 const trig_table_a[] = {
 	f2xm1, fyl2x, fptan, fpatan,
-	fxtract, fprem1, (FUNC_ST0) fdecstp, (FUNC_ST0) fincstp
+	fxtract, fprem1, fdecstp, fincstp,
 };
 
 void FPU_triga(void)

arch/x86/math-emu/reg_constant.c

@@ -108,8 +108,13 @@ static void fldz(int rc)
 
 typedef void (*FUNC_RC) (int);
 
+static void FPU_RC_illegal(int unused)
+{
+	FPU_illegal();
+}
+
 static FUNC_RC constants_table[] = {
-	fld1, fldl2t, fldl2e, fldpi, fldlg2, fldln2, fldz, (FUNC_RC) FPU_illegal
+	fld1, fldl2t, fldl2e, fldpi, fldlg2, fldln2, fldz, FPU_RC_illegal
 };
 
 void fconst(void)