[vm] Use IP address for joining IPv4 multicast on iOS
[vm] Use IP address for joining IPv4 multicast on iOS

The socket implementation has a workaround for MacOS, which is also required on iOS. @alexbatalov suggested this fix in https://github.com/dart-lang/sdk/issues/42250#issuecomment-759026385.

Verified on iPhone 11 with iOS 14.6.

Bug: http://dartbug.com/42250

TEST=Existing test base. Manually verified on iOS

Change-Id: I9942ccaf3bdd65e04842a38697eadc19feb317d9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206020
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
Commit-Queue: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
Auto-Submit: Marcin Radomski <94ae8c9b64e960905db4ad875590823243102b12@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk