[lldb] [test] Pass appropriate -L&-Wl,-rpath for libc++ on NetBSD

Pass appropriate -L and -Wl,-rpath flags pointing out to the LLVM
library directory on NetBSD.  This is necessary since clang on NetBSD
requires libc++ but it is not installed as part of the system
by default.  For the purpose of running buildbot, we want LLDB to use
just-built libc++.

Differential Revision: https://reviews.llvm.org/D58630

llvm-svn: 355502