libstdc++: Do not define constexpr assertions for C++11

libstdc++: Do not define constexpr assertions for C++11

There's no point even checking is_constant_evaluated() in C++11 mode,
because the 'if' statement used for the assertion wouldn't be valid in a
C++11 constexpr function anyway.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (__glibcxx_assert_1): Define as empty
	for C++11.