Fix building kernel gcc plugins with gcc 11.x

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).

Not for upstreaming because this has already been fixed differently
in more current kernels - but this minimal patch is far less intrusive
than backporting upstream's fix.

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>
Upstream-Status: Innapropriate [other]