llvm-config: Include -stdlib= in --cxxflags

Summary:
This was removed in r349068, but it is needed when llvm is compiled
using the non-default c++ standard library on a platform.

Reviewers: sylvestre.ledru, infinity0, mgorny, cuviper

Reviewed By: sylvestre.ledru

Subscribers: jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57859

llvm-svn: 355107