The build system is currently miss-identifying GNU/kFreeBSD as FreeBSD.

This kind of simplification is sometimes useful, but in general it's not correct. 

As GNU/kFreeBSD is an hybrid system, for kernel-related issues we want to match the
build definitions used for FreeBSD, whereas for userland-related issues we want to
match the definitions used for other systems with Glibc.

The current modification adjusts the build system so that they can be distinguished,
and explicitly adds GNU/kFreeBSD to the build checks in which it belongs.

Fixes bug #16444.

Patch by Robert Millan in the context of Debian.

llvm-svn: 185311