doc: type-punning through a union is a GNU extension for C++

doc: type-punning through a union is a GNU extension for C++

We didn't explicitly say that type-punning through a union is undefined
behavior in C++. Mention that, and that we support it as a GNU extension.

This was reported on LLVM's Discourse (forums) [0].

[0] https://discourse.llvm.org/t/ub-when-type-punning-through-unions/88527/6

gcc/ChangeLog:
	PR c++/117219

	* doc/invoke.texi (-fstrict-aliasing): Explain that type-punning
	through a union in C++ is supported as a GNU extension.