rust: Disable SSE2 to work around a rustc crash with UML

It looks like rustc is crashing when building the doctests under UML. No
idea exactly why, but there was a SIGTRAP deep within LLVM somewhere.
Disabling SSE2 seems to fix it, as noted here:
https://bestrustcrates.com/p/this-small-project-iliakonnovrust-crash/index.html

Signed-off-by: David Gow <davidgow@google.com>

scripts/generate_rust_target.rs

@@ -204,7 +204,7 @@ fn main() {
             "data-layout",
             "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128",
         );
-        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
+        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float,-sse,-sse2".to_string();
         if cfg.has("RETPOLINE") {
             features += ",+retpoline-external-thunk";
         }