Change size of reference count field in __libcpp_nmstr from 32 bits to 64 bits...

Change size of reference count field in __libcpp_nmstr from 32 bits to 64 bits for 64 bit targets.  This is controls the data layout of all exceptions defined in <stdexcept>.  This aligns the ABI with that of gcc-4.2.

llvm-svn: 161496