x86/math-emu: Fix div_Xsig() prototype

x86/math-emu: Fix div_Xsig() prototype

The third argument of div_Xsig() is the output of the division, but is marked
'const', which means the compiler is not expecting it to be updated and may
generate bad code around the call. clang-21 now warns about the pattern since
an uninitialized variable is passed into two 'const' arguments by reference:

  arch/x86/math-emu/poly_atan.c:93:28: error: variable 'argSignif' is uninitialized \
  when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
     93 |         div_Xsig(&Numer, &Denom, &argSignif);
        |                                   ^~~~~~~~~
  arch/x86/math-emu/poly_l2.c:195:29: error: variable 'argSignif' is uninitialized \
  when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
    195 |                 div_Xsig(&Numer, &Denom, &argSignif);
        |                                           ^~~~~~~~~

The implementation is in assembly, so the problem has gone unnoticed since the
code was added in the linux-1.1 days. Remove the 'const' marker here.

Fixes: e19a1bdb835c ("Import 1.1.38")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/20250807205334.123231-1-arnd@kernel.org