init/Kconfig: Only block on RANDSTRUCT for RUST

When enabling Rust in the kernel, we only need to block on the
RANDSTRUCT feature and GCC plugin. The rest of the GCC plugins
are reasonably safe to enable.

[ Originally (years ago) we only had this restriction, but we ended up
  restricting also the rest of the GCC plugins 1) to be on the safe side,
  2) since compiler plugin support could be going away in the kernel and
  3) since mixed builds are best effort so far; so I asked Neal about
  his experience enabling the other plugins -- Neal says:

    When I originally wrote this patch two years ago to get things
    working, Fedora used all the GCC plugins, so I was trying to get GCC +
    Rust to work while minimizing the delta on build differences. This was
    the combination that worked. We've been carrying this patch in the
    Asahi tree for a year now. And while Fedora does not currently have
    GCC plugins enabled because it caused issues with some third-party
    modules (I think it was the NVIDIA driver, but I'm not sure), it was
    around long enough for me to know with some confidence that it was
    fine this way.

  - Miguel ]

Signed-off-by: Neal Gompa <neal@gompa.dev>
Link: https://lore.kernel.org/r/20240731125615.3368813-1-neal@gompa.dev


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>