[test] Fix clang-test for FreeBSD and NetBSD

Lit tries to inject the shared library paths, but no action is taken
when platform.system() is not recognized, results in an environment
variable with an empty name, which is illegal.

The patch fixes this mechanism for FreeBSD and NetBSD, and gives an
warning on other platforms, so that the latecomers don't have to spend
time on debugging lit.

Thanks Zhihao Yuan for the patch!

Differential Revision: https://reviews.llvm.org/D39162

llvm-svn: 316411