[tests][go]Add -stdlib=libc++ to build GO test if LLVM is built with libc++

When libc++ is used to build LLVM libraries, these libraries have dependencies on libc++ and C++ STL signatures in these libraries are corresponding to libc++ implementation. Therefore, -stdlib=libc++ is required on the C++ compiler command for building GO tests that link with these LLVM libraries.

Reviewers: hubert.reinterpretcast, sfertile, amyk, EricWF

Reviewed By: sfertile, hubert.reinterpretcast

Subscribers: jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61900

llvm-svn: 360895