[clang] [llvm] Remove support for 3DNow!, both intrinsics and builtins. (PR #96246)

[Nathan Chancellor via cfe-commits]

nathanchance wrote:

I am seeing a lot of

```
'-3dnow' is not a recognized feature for this target (ignoring feature)
'-3dnowa' is not a recognized feature for this target (ignoring feature)
```

while building the Linux kernel (which has `-mno-3dnow` in `arch/x86/Makefile`), is this intentional behavior? I don't have a standalone reproducer yet (I am dealing with a lot of other breakage at the moment) but I wanted to inquire on whether or not that should be happening. It appears that `-mno-3dnow` is still allowed, so we could not use our `cc-option` macro, we would have to remove it only when LLVM is at least 19.0.0.

https://github.com/llvm/llvm-project/pull/96246