I made a request on the Xbyak issue tracker to allow some constructors
to be constexpr in order to avoid static constructors from needing to
execute for some of our register constants.
This request was implemented, so this updates Xbyak so that we can make
use of it.
Xbyak is currently entirely unused. Rather than carting it along, remove
it and get rid of a dependency. If it's ever needed in the future, then
it can be re-added (and likely be more up to date at that point in
time).
