[8.0][OSX] ip_mreqn struct is not supported for IPv4 Multicast (#119354)

[liveans]

Backport of #119354 to release/8.0-staging

/cc @liveans

Customer Impact

• Customer reported
• Found internally

[Select one or both of the boxes. Describe how this issue impacts customers, citing the expected and actual behaviors and scope of the issue. If customer-reported, provide the issue number.]

Regression

• Yes
• No

[If yes, specify when the regression was introduced. Provide the PR or commit if known.]

Testing

For SocketOption, it has been tested through CI, manual test runs, for actual multicast behavior we made a limited manual testing.

Risk

Low, this was broken for a while.

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

[Copilot]

Pull Request Overview

This is a backport fixing IPv4 multicast socket option handling on macOS/OSX systems. The issue stems from the fact that the ip_mreqn struct is not supported on macOS, requiring the use of IP_MULTICAST_IFINDEX socket option instead for interface index specification.

Key changes:

• Adds detection for IP_MULTICAST_IFINDEX availability during build configuration
• Implements fallback logic to use IP_MULTICAST_IFINDEX when ip_mreqn is not available
• Removes test exclusion for Apple mobile platforms since the issue is now resolved

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/native/libs/configure.cmake	Adds build-time check for IP_MULTICAST_IFINDEX socket option availability
src/native/libs/Common/pal_config.h.in	Defines configuration macro for IP_MULTICAST_IFINDEX detection
src/native/libs/System.Native/pal_networking.c	Implements fallback logic using IP_MULTICAST_IFINDEX when setting IPv4 multicast interface options
src/libraries/System.Net.Sockets/tests/FunctionalTests/SocketOptionNameTest.cs	Removes test exclusion for Apple mobile platforms