Fix TestPublicAPIHeaders test on Linux.
 -Change the build_dir variable name to lib_dir
 -Set lib_dir to the correct location on Linux
 -Set LD_EXTRAS to the actual lldb library


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@175664 91177308-0d34-0410-b5e6-96231b3b80d8