It is unfortunately not just about a “library”. The main problem is that applications and kernel will sometimes exchange memory addresses. 32-bit apps work in a 32-bit address space, but in this case the kernel will work with a 64-bit address space. This needs specific support on the kernel side to “unfold” the pointer to full 64-bit physical addresses, and “fold” them to 32-bit when giving them back to the application.
That being said, such a change would allow running legacy apps on a gcc5/x86_64 system, allowing its use as the main architecture whenever Haiku decides to do that (probably some time after R1 is out). If someone is interested in implementing this, Im sure we can find some Haiku devs to “mentor” the work (provide implementation hints and review patches).