[8u] RFR (XS): 8074836: Resolve disabled warnings for libosxkrb5

[Hohensee, Paul]

Please review this syntax-only backport to help make 8u buildable using Xcode 12.

Original issues: https://bugs.openjdk.java.net/browse/JDK-8074836 https://bugs.openjdk.java.net/browse/JDK-8074835
Original patch: http://hg.openjdk.java.net/jdk9/jdk9/jdk/rev/1f7132eec73c
8u webrev: https://cr.openjdk.java.net/~phh/8074836/webrev.8u.jdk.00/

DISABLED_WARNINGS_clang is not present in 8u, so that part of the patch is omitted. SecurityLibraries.gmk is the 8u equivalent of Lib-java.security.jgss.gmk: the former adds only a comment. The rest of the patch applies cleanly and is syntax-only. 8074835 shares the patch with 8074836.

Thanks,
Paul