build: force disable stack validation during kernel build on non-linux systems

build: force disable stack validation during kernel build on non-linux systems

The check for libelf in the kernel build is not enough, because the code that
uses libelf for stack validation is completely non-portable, as it tries to
include asm/types.h and relies on kernel types in user space.

Until this is fixed properly, the only solution is to disable this on any non
Linux build host

Signed-off-by: Felix Fietkau <nbd@nbd.name>