compiler/rustc_target/src/spec/base/apple/tests.rs: Avoid unnecessary large move

Fixes:

    $ MAGIC_EXTRA_RUSTFLAGS=-Zmove-size-limit=4096 ./x test compiler/rustc_target
    error: moving 6216 bytes
      --> compiler/rustc_target/src/spec/base/apple/tests.rs:17:19
       |
    17 |     for target in all_sim_targets {
       |                   ^^^^^^^^^^^^^^^ value moved from here
       |
       = note: The current maximum size is 4096, but it can be customized with the move_size_limit attribute: `#![move_size_limit = "..."]`
       = note: `-D large-assignments` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(large_assignments)]`

compiler/rustc_target/src/spec/base/apple/tests.rs

@@ -14,7 +14,7 @@ fn simulator_targets_set_abi() {
         aarch64_apple_watchos_sim::target(),
     ];
 
-    for target in all_sim_targets {
+    for target in &all_sim_targets {
         assert_eq!(target.abi, "sim")
     }
 }