Support of C11's spelling of alignof

[jjst]

C11 standardized the use of alignof through the _Alignof operator, see http://en.cppreference.com/w/c/language/_Alignof. language-c will correctly parse the following spellings: alignof, __alignof__, and __alignof, but throws an error when parsing the new _Alignof (which the alignof macro gets expanded to in C11).