[libc 1.0] Add IN6ADDR_ANY_INIT, IN6ADDR_LOOPBACK_INIT, in6addr_any, in6addr_loopback

[safinaskar]

This is not breaking change.

Note that this PR adds global variables (statics) in6addr_any and in6addr_loopback. But this is okay, because:

• We already have global variables in crate libc: https://docs.rs/libc/0.2.154/aarch64-apple-darwin/libc/index.html#statics
• ctest2 knows how to test global variables: https://docs.rs/ctest2/0.4.8/ctest2/struct.TestGenerator.html#method.skip_static

This PR closes https://github.com/rust-lang/libc/issues/1950

Also I suggest merging this PR for ctest2: https://github.com/JohnTitor/ctest2/pull/56 , it removes annoying warnings produced by ctest2 in presence of statics. And also fixes CI for ctest2. I recommend merging https://github.com/JohnTitor/ctest2/pull/56 , but this is not required