Support mb sequences across calls to mb*to*wcs* functions

Bug: 13077905
Change-Id: I5abdc7cc3c27c109b7900c94b112f18a95c35763

libc/include/wchar.h

@@ -41,11 +41,9 @@ __BEGIN_DECLS
 
 typedef __WINT_TYPE__  wint_t;
 typedef struct {
-#ifdef __LP32__
-  int dummy;
-#else
-  // 8 bytes should be enough to support at least UTF-8
-  char __reserved[8];
+  uint8_t __seq[4];
+#ifdef __LP64__
+  char __reserved[4];
 #endif
 } mbstate_t;