Add initial NetBSD support in lldbsuite/test/lldbtest.py

Summary:
Add new functions:

  - expectedFailureNetBSD()
  - expectedFlakeyNetBSD()
  - skipIfNetBSD()

Add new NetBSD entry in:

  - getPlatform()
  - getHostPlatform()

Assume that libc++ is installed and use the GNU toolchain

Reviewers: joerg, emaste, tfiala, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D15262

llvm-svn: 254948