This uses a custom fork of `hwy/targtes.cpp` that uses an extern
function written in Zig to use Zig's standard CPU detection to avoid
a dependency on Apple SDK headers.
This is on the path to removing Apple SDK requirements to build
libghostty-vt, but will require a lot more work outside of this. The goal
is to get this out of our external dependencies first and then we can
work on removing the internal side.