JDK-8022263: use same Clang warnings on BSD as on Linux - Any *BSD testers?

[Andrew Hughes]

I'm looking at https://bugs.openjdk.java.net/browse/JDK-8022263, which
is a clean backport and a pre-requisite for some later changes.

This is a fairly simple patch, basically synchronising the warning logic
with the GNU/Linux platform. The patch builds fine with GCC 7 on
GNU/Linux, but it would be good if someone could check these additional
flags don't break the build on *BSD.

Generally, it'd also be good to know who is testing 8u on which
platforms for fixes such as this.

Thanks,
-- 
Andrew :)

Senior Free Java Software Engineer
Red Hat, Inc. (http://www.redhat.com)

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