kernel: don't disable stack validation in cross builds

kernel: don't disable stack validation in cross builds

c3e31b6a9b04 (2019-07-03), 5f8e5872406d (2019-10-02), and 802bfe080678
(2022-10-20) disabled kernel stack validation when the build host was
not running Linux, as the objtool kernel build tool required for stack
validation was not portable to other build host environments. This is no
longer true, and it is not necessary to attempt to disable kernel stack
validation any longer.

Signed-off-by: Mark Mentovai <mark@mentovai.com>
Link: https://github.com/openwrt/openwrt/pull/23612
Signed-off-by: Robert Marko <robimarko@gmail.com>