libstdc++: Ensure __glibcxx_assert_fail has default visibility

libstdc++: Ensure __glibcxx_assert_fail has default visibility

This ensures there's no linker error if libstdc++ headers are included
following a pragma that sets hidden visibility.

Similarly for std::__terminate, which is always-inline so shouldn't
matter, but it's not wrong to do this anyway.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (__glibcxx_assert_fail): Add visibility
	attribute.
	(__terminate): Likewise.