When saving callee saved registers while doing an upcall, we have to follow the ABI, through ABIDescriptor, to determine which registers should be saved.

However, on certain ABIs not the entire register needs to be saved. For instance, on aarch64 only the first 8 bytes of a FloatRegister are saved.

This is currently hard-coded in the native code.

Also, whether and how status register such as mxcsr and fpcsr should be be preserved is hard-coded.

How these register should be saved should really be communicated through ABIDescriptor instead.

Doing so also allows other ABIs to only save part of a callee-saved register, and possibly save space/time when doing an upcall.