[CMake] Add option to switch default C++ stdlib

With this option one can optionally override the architecture dependent
default library to use if no -stdlib= is provided on compiler invocation.

Differential Revision: http://reviews.llvm.org/D15920

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260662 91177308-0d34-0410-b5e6-96231b3b80d8