[gardening] Guard use of lib atomic so  it's not used on ios builds.

This is follow-up to e78e0dd2610f6d67419bb65ceeabfaf0c405e6ab.

TEST=ci

Change-Id: I089299076b115ee384bc2f485a01045367bf78ef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/222026
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
diff --git a/runtime/vm/BUILD.gn b/runtime/vm/BUILD.gn
index 1b03265..30f1303 100644
--- a/runtime/vm/BUILD.gn
+++ b/runtime/vm/BUILD.gn
@@ -56,7 +56,7 @@
     if (!is_android) {
       libs += [ "pthread" ]
     }
-    if (!is_mac) {
+    if (!is_mac && !is_ios) {
       libs += [ "atomic" ]
     }
   }