ubsan: Disable signed integer overflow sanitizer on GCC < 8

For opting functions out of sanitizer coverage, the "no_sanitize"
attribute is used, but in GCC this wasn't introduced until GCC 8.
Disable the sanitizer unless we're not using GCC, or it is GCC
version 8 or higher.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403110643.27JXEVCI-lkp@intel.com/


Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>