unittests/SupportTests/Initialize.MultipleThreads: Enable pthread_attr_setstack(3) only on Linux.

I got blamed on darwin11;
unittests/Support/ManagedStatic.cpp:35: error: 'pthread_attr_setstack' was not declared in this scope

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173355 91177308-0d34-0410-b5e6-96231b3b80d8