Fix building the kernel's gcc plugins with gcc 11.x - gcc 11.x needs
constexpr (hence -std=gnu++11 or higher), and the check for gcc
plugin support needs modifications (or simple disabling, given in
our context, we know gcc has plugin support).

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>