Added apxf target feature test

Author: madhav-madhusoodanan

tests/ui/feature-gates/feature-gate-apx-target-feature.rs

@@ -0,0 +1,6 @@
+//@ only-x86_64
+#[target_feature(enable = "apxf")]
+//~^ ERROR: currently unstable
+unsafe fn foo() {}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-apx-target-feature.stderr

@@ -0,0 +1,13 @@
+error[E0658]: the target feature `apxf` is currently unstable
+  --> $DIR/feature-gate-apx-target-feature.rs:2:18
+   |
+LL | #[target_feature(enable = "apxf")]
+   |                  ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #139284 <https://github.com/rust-lang/rust/issues/139284> for more information
+   = help: add `#![feature(apx_target_feature)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.