Show the complete LLVM intrinsic name when it is unimplemented

[Here](https://github.com/rust-lang/rustc_codegen_gcc/blob/99780df8add4790dd838d5172257d6e66124597a/src/intrinsic/archs.rs#L10176).
Currently, the prefix is stripped in the error message, so we see `avx512.uitofp.round.v16f16.v16i16` instead of `llvm.x86.avx512.uitofp.round.v16f16.v16i16`.