Add initial CMake glue for the NetBSD platform

Summary:
These changes aren't everything what is needed for the CMake target, but it's significantly approaching it.

These changes shouldn't effect the build process on other platforms.

Patch by Kamil Rytarowski, thanks!

Reviewers: joerg, brucem

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13711

llvm-svn: 251164