xbyak is intended to be installed in /usr/local/include/xbyak.
Since we desire not to install xbyak before using it, we copy the headers
to the appropriate directory structure and use that instead
Co-authored-by: merry <git@mary.rs>
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.
