PR libstdc++/88119 use alignof in std::alignment_of, not __alignof__

PR libstdc++/88119 use alignof in std::alignment_of, not __alignof__

Now that __alignof__ and alignof sometimes disagree it matters which one
we use. The standard says that std::alignment_of<T>::value equals
alignof(T), so we need to use that.

Change the only uses of alignment_of to use __alignof__ to avoid a
change in alignment.

	PR libstdc++/88119
	* include/ext/aligned_buffer.h (__aligned_membuf): Add comment.
	(__aligned_buffer): Use __alignof__ instead of std::alignment_of.
	* include/std/type_traits (alignment_of): Use alignof instead of
	__alignof__.
	* testsuite/20_util/alignment_of/value.cc: Fix test to check values
	match alignof not __alignof__, as required by the standard.

From-SVN: r266613