Fix the new _ALIGNAS_TYPE per instructions supplied by Eli Friedman.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@157765 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config b/include/__config
index c104a3f..369417c 100644
--- a/include/__config
+++ b/include/__config
@@ -148,7 +148,7 @@
 #  define _ALIGNAS_TYPE(x) alignas(x)
 #  define _ALIGNAS(x) alignas(x)
 #else
-#  define _ALIGNAS_TYPE(x) __attribute__((__aligned__))
+#  define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
 #  define _ALIGNAS(x) __attribute__((__aligned__(x)))
 #endif