radv: restrict exported symbols with static llvm

radv: restrict exported symbols with static llvm

Like the gallium --version-script magic but for radv.

The long term goal is to make LLVM support optional, remove it even, so
let's keep the hunk in an if block.

v2: fold if checks (Eric)
v3 (Tomeu): Remove spaces within [] (Dylan)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl> (v1)
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6203>