rust: x86: remove -3dnow{,a} from target features

LLVM 19 is dropping support for 3DNow! in commit f0eb5587ceeb ("Remove
support for 3DNow!, both intrinsics and builtins. (#96246)"):

    Remove support for 3DNow!, both intrinsics and builtins. (#96246)

    This set of instructions was only supported by AMD chips starting in
    the K6-2 (introduced 1998), and before the "Bulldozer" family
    (2011). They were never much used, as they were effectively superseded
    by the more-widely-implemented SSE (first implemented on the AMD side
    in Athlon XP in 2001).

    This is being done as a predecessor towards general removal of MMX
    register usage. Since there is almost no usage of the 3DNow!
    intrinsics, and no modern hardware even implements them, simple
    removal seems like the best option.

Thus we should avoid passing these to the backend, since otherwise we
get a diagnostic about it:

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

We could try to disable them only up to LLVM 19 (not the C side one,
but the one used by `rustc`, which may be built with a range of
LLVMs). However, to avoid more complexity, we can likely just remove
them altogether. According to Nikita [2]:

> I don't think it's needed because LLVM should not generate 3dnow
instructions unless specifically asked to, using intrinsics that Rust
does not provide in the first place.

Thus do so, like Rust did for one of their builtin targets [3].

For those curious: Clang will warn only about trying to enable them
(`-m3dnow{,a}`), but not about disabling them (`-mno-3dnow{,a}`), so
there is no change needed there.

Cc: Nikita Popov <github@npopov.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: x86@kernel.org
Link: llvm/llvm-project@f0eb558 [1]
Link: rust-lang/rust#127864 (comment) [2]
Link: rust-lang/rust#127864 [3]
Closes: Rust-for-Linux#1094
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

scripts/generate_rust_target.rs

@@ -162,7 +162,7 @@ fn main() {
             "data-layout",
             "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
         );
-        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
+        let mut features = "-mmx,+soft-float".to_string();
         if cfg.has("MITIGATION_RETPOLINE") {
             features += ",+retpoline-external-thunk";
         }
@@ -179,7 +179,7 @@ fn main() {
             "data-layout",
             "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128",
         );
-        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
+        let mut features = "-mmx,+soft-float".to_string();
         if cfg.has("MITIGATION_RETPOLINE") {
             features += ",+retpoline-external-thunk";
         }