Chore: add x86_amx_intrinsics feature flag to core/lib.rs and rem…

…ove `issue-120720-reduce-nan.rs`

core/src/lib.rs

@@ -264,6 +264,7 @@
 #![feature(sse4a_target_feature)]
 #![feature(tbm_target_feature)]
 #![feature(wasm_target_feature)]
+#![feature(x86_amx_intrinsics)]
 // tidy-alphabetical-end
 
 // allow using `core::` in intra-doc links