meson: Error out if building nouveau and using LLVM without rtti

meson: Error out if building nouveau and using LLVM without rtti

Nouveau requires rtti. Often LLVM is configured without rtti, and code
with and without cannot be linked safely. Lets just error out if nouveau
is requested and llvm is built without rtti.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109202
Fixes: c5a97d658ec19cc02719d7f86c1b0715e3d9ffc4
       ("meson: fix builds against LLVM built without rtti")
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit a2596450ac7330c8965c819491038fb1ad454333)