runtime: workaround GCC 4.8 libstdc++ C++11 conformance

libstdc++ included with GCC 4.8 does not define `std::max_align_t` as
required by the C++11 specification.  As a workaround, explicitly create
the definition locally.  This was fixed in GCC 4.9 and later.

stdlib/public/runtime/Casting.cpp

@@ -44,6 +44,11 @@
 #include <cstring>
 #include <type_traits>
 
+#if defined(__GLIBCXX__) && __GLIBCXX__ < 20160726
+#include <stddef.h>
+namespace std { using ::max_align_t; }
+#endif
+
 using namespace swift;
 using namespace swift::hashable_support;
 using namespace metadataimpl;