Preliminary NetBSD support

Summary:
This adds platform code without the cmake/gmake glue to the existing infrastructure.

The missing and incompatibility ptrace(2) bits (existing in FreeBSD) are under active research and development and will be submitted once verified to work.

This code was tested to build and run on NetBSD-current/amd64.

Proper build scripts will be integrated separately as a new commit.

Reviewers: joerg

Subscribers: tfiala, brucem, labath, emaste, lldb-commits

Differential Revision: http://reviews.llvm.org/D13334

llvm-svn: 250146