> IIRC GCC and Clang lets character types alias to any type.

It is always legal to access the memory representation of any object as an array of characters. The other way around (interpreting an array of characters as a T, even though it does not have effective type T) is not.

> Otherwise glibc’s malloc also doesn’t abide to strict aliasing.

It may not have to. From the point of view of C, malloc is special because it is part of the implementation. The compiler is free to handle UB as it sees fit. In particular, it can decide that aliasing has different semantics in malloc.c than outside it.