libstdc++: Micro-optimization for std::addressof

libstdc++: Micro-optimization for std::addressof

Currently std::addressof calls std::__addressof which uses
__builtin_addressof. This leads to me prefering std::__addressof in some
code, to avoid the extra hop. But it's not as though the implementation
of std::__addressof is complicated and reusing it avoids any code
duplication.

So let's just make std::addressof use the built-in directly, and then we
only need to use std::__addressof in C++98 code. (Transitioning existing
uses of std::__addressof to std::addressof isn't included in this
change.)

The front end does fold std::addressof with -ffold-simple-inlines but
this change still seems worthwhile.

libstdc++-v3/ChangeLog:

	* include/bits/move.h (addressof): Use __builtin_addressof
	directly.