x86/asm: Avoid unneeded __div64_32 function definition

The __div64_32() function is provided for 32-bit architectures that
don't have a custom do_div() implementation. x86_32 has one, and
does not use the header file that declares the function prototype,
so the definition causes a W=1 warning:

  lib/math/div64.c:31:32: error: no previous prototype for '__div64_32' [-Werror=missing-prototypes]

Define an empty macro to prevent the function definition from getting
built, which avoids the warning and saves a little .text space.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230803082619.1369127-4-arnd@kernel.org