Expand Serialization Register

[CodyFagley]

AArch64's NEON registers allow XCS to have more integer registers for general-purpose computing.

However, this shift requires that more registers are tracked, so the Serialization register will need to be modified to handle the increase in addressable registers.

The first register that should be repurposed for additional serialization space is v17. Depending on needs, v31 (OS Parameters Register) may also be available.