[OpenMP] Base support for target directive codegen on NVPTX device.

Summary:
This patch adds base support for codegen of the target directive on the NVPTX device.

Reviewers: ABataev

Differential Revision: http://reviews.llvm.org/D17877

Reworked test case after buildbot failure on windows.
Updated patch to integrate r263837 and test case nvptx_target_firstprivate_codegen.cpp.

llvm-svn: 264018