[libcxx] Whitelist inclusion of sysctl.h instead of blacklisting

Instead of excluding all known operating systems that are not derived from BSD,
I now include all operating systems that claim to be derived from BSD.
Hopefully, that will make it so that this check doesn't need to change for
every new operating system that comes along.

http://reviews.llvm.org/D16634

llvm-svn: 259193