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