Fixed memory management issues introduced by revision 157507.

A local std::string was being filled in and then the function would return "s.c_str()".
A local StreamString (which contains a std::string) was being filled in, and essentially also returning the c string from the std::string, though it was in a the StreamString class.

The fix was to not do this by passing a stream object into StringList::Join() and fix the "arch_helper()" function to do what it should: cache the result in a global.

llvm-svn: 157519