<linux/overflow.h>: Don't use __has_builtin().

GCC only added support for __has_builtin in GCC 10. However, all
supported versions of GCC and clang include these builtins so just use
them unconditionally.

This fixes the build with GCC 9.

Reviewed by: manu, hselasky, imp
Differential Revision: https://reviews.freebsd.org/D31942