[build] Add missing -latomic.

Newer versions of gcc seem to require this dependency to be explicit.

TEST=ci
Bug: https://github.com/dart-lang/sdk/issues/38587
Change-Id: Ie27d901b639505091d86abeec51146c65bee0ef1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/221886
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/vm/BUILD.gn

@@ -56,6 +56,9 @@ config("libdart_vm_config") {
     if (!is_android) {
       libs += [ "pthread" ]
     }
+    if (!is_mac) {
+      libs += [ "atomic" ]
+    }
   }
 }