x86/its: Fix undefined reference to cpu_wants_rethunk_at()

Below error was reported in a 32-bit kernel build:

  static_call.c:(.ref.text+0x46): undefined reference to `cpu_wants_rethunk_at'
  make[1]: [Makefile:1234: vmlinux] Error

This is because the definition of cpu_wants_rethunk_at() depends on
CONFIG_STACK_VALIDATION which is only enabled in 64-bit mode.

Define the empty function for CONFIG_STACK_VALIDATION=n, rethunk mitigation
is anyways not supported without it.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 5d19a057 ("x86/its: Add support for ITS-safe return thunk")
Link: https://lore.kernel.org/stable/0f597436-5da6-4319-b918-9f57bde5634a@roeck-us.net/


Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>