Auto merge of #116093 - RalfJung:link_llvm_intrinsics, r=oli-obk

make link_llvm_intrinsics and platform_intrinsics features internal

These are both a lot like `feature(intrinsics)`, just slightly different syntax, so IMO it should be treated the same (also in terms of: if you get ICEs with this feature, that's on you -- we are not doing "nice" type-checking for intrinsics).

Author: bors