arch: Add _wchar_t typedef like other basic types

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

arch/misoc/include/types.h

@@ -64,6 +64,12 @@ typedef unsigned long long _uint64_t;
 typedef _int64_t           _intmax_t;
 typedef _uint64_t          _uintmax_t;
 
+#if defined(__WCHAR_TYPE__)
+typedef __WCHAR_TYPE__     _wchar_t;
+#else
+typedef int                _wchar_t;
+#endif
+
 /* A size is 4 bytes */
 
 #if defined(__SIZE_TYPE__)